Fetal development

For the first 12 hours after conception, the fertilized egg remains a single cell. After 30 hours or so, it divides from one cell into two. Some 15 hours later, the two cells divide to become four. And at the end of 3 days, the fertilized egg cell has become a berry-like structure made up of 16 cells. This structure is called a morula, which is Latin for mulberry.

During the first 8 or 9 days after conception, the cells that will eventually form the embryo continue to divide. At the same time, the hollow structure in which they have arranged themselves, called a blastocyst, is slowly carried toward the uterus by tiny hair-like structures in the fallopian tube, called cilia.

The blastocyst, though only the size of a pinhead, is actually composed of hundreds of cells. During the critically important process of implantation, the blastocyst must attach itself to the lining of the uterus or the pregnancy will not survive.

If we take a closer look at the uterus, you can see that the blastocyst actually buries itself in the lining of the uterus, where it will be able to get nourishment from the mother’s blood supply.

During the fifth month, a baby’s outer ears are almost fully developed. Ear formation starts from a few small lumps during the second month. Let’s go and take a look.

Here we see a baby during the fifth week of development. Those small bulges are called branchial arches. During month three, the branchial arches formed the lower face and neck. The ears developed from them, too, between those two branchial arches. Let’s take a closer look. In the fifth week, they were smooth. But a week after that, tiny bumps called auricular hillocks formed on each branchial arch. Now here’s the fascinating part. Watch what happens from the sixth week until the end of the fifth month...As you can see, the auricular hillocks grew and joined together to form the baby’s outer ears.

During the fifth month, the inner and middle part of the baby’s ears are also developing, but they won’t be completely finished until birth. And here’s what they’ll look like at that time.

The embryo’s heart is the first organ that forms in its tiny body, and like most complex instruments, it begins with some simple structures.

Let’s go back to 18 days after conception...Looking in the mother’s uterus, you can see the embryo surrounded by its yolk sac and amnion. Let’s take a look inside.

Here’s a diagram of the embryo seen from a side view. Right now, it’s about the size of a raisin. There’s the head region and that red-colored area slightly above it contains two tubes that will form the embryo’s heart. Here’s what the tubes look like from a front view.

On day 21, we see that the primitive heart tubes have moved below the embryo’s developing head region. And by day 22, the tubes have fused together, and have moved to the area that will eventually be our embryo’s thoracic, or chest cavity. It’s also about this time that the heart begins to beat for the first time...

Here’s what it looks like from the front.

Now let’s go back to day 18 and watch this happen from a different viewpoint. Here are two tubes in our embryo’s chest region seen from a front view. Watch this... Over the next two days, these tubes fuse together.

Here’s another amazing part: the tube now starts bending and twisting and over the next 8 days it forms a simple version of the heart.

By the time the embryo becomes a fetus at two months, the heart bears a close resemblance to what it will look like after the baby’s born. But the resemblance is only superficial. On the inside of the heart, things are much different in both form and function.

Here’s a newborn heart on the left. Let’s take a closer look. There’s the right atrium right ventricle, left atrium and left ventricle. The two major blood vessels are the aorta and the pulmonary artery.

The pathway of blood in the newborn heart works like this: oxygen-poor blood from the body enters the right atrium, then goes to the right ventricle. From the right ventricle, the blood is pumped to the lungs where it becomes oxygen rich. Then the blood flows back to the heart filling the left atrium and from there on to the left ventricle. The left ventricle pumps the oxygen rich blood through the aorta, which carries it to the rest of the newborn’s body.

You can see the fetal heart has the same basic components as the newborn heart, but there are a couple important differences. Because the placenta is providing all of the oxygen the fetus requires, its lungs are not needed to perform this task, and therefore much of the fetus’ blood is detoured away from the lungs through two openings or connections. They are the foramen ovale, which connects the right and left atria, and the ductus arteriosus which connects the aorta and the pulmonary artery.

As blood enters the heart into the right atrium some of the blood flows into the right ventricle as in the newborn, but also notice that some blood flows directly into the left atrium through the foramen ovale. This blood will pass directly into the left ventricle and be pumped out to the body without ever having gone to the lungs. In addition, some of the blood that did enter the right ventricle, and would normally go to the lungs, never reaches the lungs.

Here lets watch. As blood is being pumped out of the right ventricle towards the lungs through the pulmonary artery, some of that blood escapes into the aorta through the ductus arteriosus, bypassing the lungs as it does. These two important connections will remain open up until the time of birth.

Within thirty minutes after the baby’s first breath, the ductus arteriosus will completely close, and the flap of the foramen ovale will shut off like a valve. This happens because of an increase in pressure on the left side of the heart, and a decrease on the right side. These changes in the heart anatomy cause the blood to flow to the lungs, which will take over their lifelong job of supplying oxygen to the body.

It’s incredible to think that this complex organ started off as a couple of tubes only 2 1/2 weeks ago.

You might not be aware of this, but during its early development a fetus looks remarkably like something from the dawn of time.

Here, let’s take a closer look. There’s a human fetus’s head during the first month of development, when it was still an embryo. Its face starts as a series of paired tissue mounds called branchial arches.

Let’s take a look from the front. The embryo’s face actually forms from the first branchial arch, along with the area just above it. The forehead and nose form from this area. These areas will form the cheekbones, and these lower areas will form the lower jaw. And this area will form the mouth. At 28 days of development, you can see the lower jaw, which has fused together from the branchial arches. The thickenings you see here will eventually form the nostrils.

By day 31, you can see the nostrils have started to form. And, quite remarkably, the eyes have now appeared on each side of the head.

Two days later, the nostrils have moved toward the center of the face. You can also see that as the ears begin to form, they are positioned in a pretty odd location. But don’t worry, they will move.

At 35 days, the nostrils are even closer together, and we can see more of the eyes. At 40 days, the baby has developed eyelids, and the nose looks much more developed.

Here he is at 48 days and he’s looking pretty darn good. The nasal swellings have joined in the center of the face, and the eyes have moved to the front of the head.

Three weeks later, the fetus looks more human than ever. After that, its face continues to develop more typical proportions right up until the time of its birth.

Let’s look at the entire process again...

As you can see, the development of the face is a fascinating process that has some very dramatic changes taking place in a relatively short amount of time.

The most critical stage of development for the embryo’s nervous system is the third and fourth weeks of pregnancy. Starting with the uterus, let’s enlarge the area where the embryo has implanted. Here’s the embryo implanted in the uterus on day 14. You can see that it lies within the wall of the uterus, and is covered by a single layer of cells.

Take a moment to get oriented. There’s the yolk sac, which makes blood cells for the embryo, and the amnion, which surrounds and protects the embryo, and the blood vessels that will help form the placenta. Also notice that the embryo is connected to the uterus by a small connecting stalk, which will eventually become the umbilical cord.

If we rotate it to the left we’ll get a better view of the back of the embryo, where we can see the brain and spinal cord develop.

On day 14, the embryo looks like a little disc. The first part of the nervous system that forms is an indentation called the neural groove. Over the next seven days, the groove deepens as the cells around it form ridges called neural folds.

By day 27 we see that the neural folds wrap around the neural groove and form the neural tube. The neural fold will become the spinal cord. Those bundles of cells that look like building blocks are called somites. They form the vertebral column, or backbone. They also help form the ribs, and the muscles of the neck, arms, and legs.

Now let’s watch it again without interruption…

If we rotate the embryo again to the left, we can see the areas that will eventually become the brain and spinal cord. The embryo’s nervous system is particularly vulnerable at this stage of development, so an expectant mother should be careful about avoiding any substances that could potentially harm it.

Many people have mistaken ideas about how a growing embryo eats and breathes in the uterus.

From the earliest stages of its development, the growing embryo requires nutrition and oxygen, and a disposal system for the waste products of its own metabolism. All of this is accomplished by the placenta, which allows the growing embryo to eat and breathe while in the mother's uterus.

To get some perspective on how the placenta began, let's go back to Day 8. This hollow ball of cells moving through the uterus is the blastocyst, searching for an implantation site. Here you see its outer layer beginning to extend out and implant in the uterine lining, searching for the uterine blood vessels that would nourish it throughout the pregnancy.

As it went deeper, a single layer of cells from the mother's uterine lining surrounded it, so that it would be protected from harm. On Day 9, as it grew larger and more complex, the blastocyst became an embryo. Here it's about the size of a pinhead.

Also on Day 9, the outer layer of the embryo developed spaces called lacunae. The lacunae filled up with blood from the mother's uterine lining.

On Day 13, small projections from the embryo's chorionic layer reached out into the uterine lining. The chorionic layer is one of the membranes that surround the embryo and help it implant.

On Days 15 through 21, blood vessels began to form beneath this chorionic layer.

Around Day 21, the embryo's blood stream and the mother's blood stream were in such close contact that nutrients and oxygen could cross from mother to embryo. This was how the embryo first got its food and air from the mother, and technically this is when the placenta began to function.

Let's magnify this area so you can see what we're talking about. Here you see a vein and an artery from the embryo in close contact with the blood in the mother's uterine lining. Inside the blood vessels, you can also see red blood cells, which carry oxygen.

The two blood streams are separated by a thin collection of tissues in the placenta called the blood barrier. This barrier permits small particles like nutrients and oxygen to pass from the mother to the embryo, and allows waste products to pass from the embryo back to the mother. The blood barrier also prevents many large or potentially harmful particles from entering the embryo's blood stream. Notice that the red blood cells do not cross from the mother's blood stream to the embryo's.

You may be wondering how a mother's blood cells could be harmful to her growing baby, and why it's important to keep the two blood streams separate. If the mother's blood type is RH negative, and her embryo's blood type is RH positive, then the mother's antibodies would treat the embryo as an invading foreign organism, and try to destroy it.

Now you can see why the placenta and its blood barrier are important for supplying the growing embryo with nutrition and oxygen, removing its waste products, and preventing harmful substances from getting into its blood stream.

A baby’s sex is determined at the time of conception. When the baby is conceived, a chromosome from the sperm cell, either X or Y, fuses with the X chromosome in the egg cell, determining whether the baby will be female or male. Two X’s means the baby will be a girl, and XY means it will be a boy.

But even though gender is determined at conception, the fetus doesn’t develop its external sexual organs until the fourth month of pregnancy.

Let’s go to seven weeks after conception. You can see from the front that the fetus appears to be sexually indifferent, looking neither like a male or a female.

Over the next five weeks, the fetus begins producing hormones that cause its sex organs to grow into either male or female organs. This process is called sexual differentiation.

We don’t know what sex this fetus is yet, so we’ll have to be hypothetical here....Now, if the fetus is a male, it will produce hormones called androgens, which will cause his sexual organs to form like this.

On the other hand, a female fetus would not produce androgens; she would produce estrogens… so her sex organs would form like this.

Now let’s take a look at something you may have missed. At seven weeks, the sex organs of a male and female look identical. Let’s add some color to see what happens during sexual differentiation. Keep your eye on the genital tubercle.

See that? The genital tubercle formed the penis in the male, and the clitoris in the female.

The penis and clitoris are called sexual analogs because they originate from the same structure.

A baby’s skeleton begins as fragile membranes and cartilage, but after three months, the membranes and cartilage start turning into bone, providing protection for the internal organs, and a solid framework for the muscles.

Late in the second month of fetal development, a fetus’ skeleton is made up of thin membranes, which are about the thickness of paper tissue, and soft, flexible cartilage, like the kind you find in your ear. Over time, both types of tissue will turn into bone in a process called ossification.

Ossification occurs in two ways...the first is when membranes turn into bone.

If we look at a fetus during the third month, we can see that the membranes on the side and back of the skull are starting to ossify. That means that the bone tissue is slowly growing over the area where the membranes once existed. Eventually, these bone plates will grow together forming the cranial cavity which protects the brain.

As the baby’s development is close to birth, you can see the bones of the skull still have gaps between them. These gaps, called fontanelles, allow room for the baby’s brain to grow, and also enable the head to be compressed during delivery.

The fontanelles will remain open until the end of the second year. And even though they’re commonly known as the baby’s soft spot, the fontanelles are actually about the thickness and strength of a piece of canvas. Which kind of makes them a soft, but tough, spot.

The bones of the skull won’t stop growing until a child reaches adulthood. That’s when the joints between the bones, called the sutures, will fuse together.

Now let’s go back once again and watch the second type of ossification when cartilage turns into bone. This time we’ll look at the hand. Most of the bones of the skeleton, like the arms, legs, ribs, fingers, and backbone, start off as cartilage.

We can get a good idea of how cartilage turns into bone by looking at this portion of the hand. Here’s what it looks like on the inside.

From the second month until the end of the third month, remarkable changes take place. Watch the middle of the cartilage: both the inside and the outside turn into bone, or ossifies.

This is how the bones will continue to grow until adulthood---from the middle of the bone outward. That way they can continue to increase in their length and width.

For the first 12 hours after conception, the fertilized egg remains a single cell. After 30 hours or so, it divides from one cell into two. Some 15 hours later, the two cells divide to become four. And at the end of 3 days, the fertilized egg cell has become a berry-like structure made up of 16 cells. This structure is called a morula, which is Latin for mulberry.

During the first 8 or 9 days after conception, the cells that will eventually form the embryo continue to divide. At the same time, the hollow structure in which they have arranged themselves, called a blastocyst, is slowly carried toward the uterus by tiny hair-like structures in the fallopian tube, called cilia.

The blastocyst, though only the size of a pinhead, is actually composed of hundreds of cells. During the critically important process of implantation, the blastocyst must attach itself to the lining of the uterus or the pregnancy will not survive.

If we take a closer look at the uterus, you can see that the blastocyst actually buries itself in the lining of the uterus, where it will be able to get nourishment from the mother’s blood supply.

During the fifth month, a baby’s outer ears are almost fully developed. Ear formation starts from a few small lumps during the second month. Let’s go and take a look.

Here we see a baby during the fifth week of development. Those small bulges are called branchial arches. During month three, the branchial arches formed the lower face and neck. The ears developed from them, too, between those two branchial arches. Let’s take a closer look. In the fifth week, they were smooth. But a week after that, tiny bumps called auricular hillocks formed on each branchial arch. Now here’s the fascinating part. Watch what happens from the sixth week until the end of the fifth month...As you can see, the auricular hillocks grew and joined together to form the baby’s outer ears.

During the fifth month, the inner and middle part of the baby’s ears are also developing, but they won’t be completely finished until birth. And here’s what they’ll look like at that time.

The embryo’s heart is the first organ that forms in its tiny body, and like most complex instruments, it begins with some simple structures.

Let’s go back to 18 days after conception...Looking in the mother’s uterus, you can see the embryo surrounded by its yolk sac and amnion. Let’s take a look inside.

Here’s a diagram of the embryo seen from a side view. Right now, it’s about the size of a raisin. There’s the head region and that red-colored area slightly above it contains two tubes that will form the embryo’s heart. Here’s what the tubes look like from a front view.

On day 21, we see that the primitive heart tubes have moved below the embryo’s developing head region. And by day 22, the tubes have fused together, and have moved to the area that will eventually be our embryo’s thoracic, or chest cavity. It’s also about this time that the heart begins to beat for the first time...

Here’s what it looks like from the front.

Now let’s go back to day 18 and watch this happen from a different viewpoint. Here are two tubes in our embryo’s chest region seen from a front view. Watch this... Over the next two days, these tubes fuse together.

Here’s another amazing part: the tube now starts bending and twisting and over the next 8 days it forms a simple version of the heart.

By the time the embryo becomes a fetus at two months, the heart bears a close resemblance to what it will look like after the baby’s born. But the resemblance is only superficial. On the inside of the heart, things are much different in both form and function.

Here’s a newborn heart on the left. Let’s take a closer look. There’s the right atrium right ventricle, left atrium and left ventricle. The two major blood vessels are the aorta and the pulmonary artery.

The pathway of blood in the newborn heart works like this: oxygen-poor blood from the body enters the right atrium, then goes to the right ventricle. From the right ventricle, the blood is pumped to the lungs where it becomes oxygen rich. Then the blood flows back to the heart filling the left atrium and from there on to the left ventricle. The left ventricle pumps the oxygen rich blood through the aorta, which carries it to the rest of the newborn’s body.

You can see the fetal heart has the same basic components as the newborn heart, but there are a couple important differences. Because the placenta is providing all of the oxygen the fetus requires, its lungs are not needed to perform this task, and therefore much of the fetus’ blood is detoured away from the lungs through two openings or connections. They are the foramen ovale, which connects the right and left atria, and the ductus arteriosus which connects the aorta and the pulmonary artery.

As blood enters the heart into the right atrium some of the blood flows into the right ventricle as in the newborn, but also notice that some blood flows directly into the left atrium through the foramen ovale. This blood will pass directly into the left ventricle and be pumped out to the body without ever having gone to the lungs. In addition, some of the blood that did enter the right ventricle, and would normally go to the lungs, never reaches the lungs.

Here lets watch. As blood is being pumped out of the right ventricle towards the lungs through the pulmonary artery, some of that blood escapes into the aorta through the ductus arteriosus, bypassing the lungs as it does. These two important connections will remain open up until the time of birth.

Within thirty minutes after the baby’s first breath, the ductus arteriosus will completely close, and the flap of the foramen ovale will shut off like a valve. This happens because of an increase in pressure on the left side of the heart, and a decrease on the right side. These changes in the heart anatomy cause the blood to flow to the lungs, which will take over their lifelong job of supplying oxygen to the body.

It’s incredible to think that this complex organ started off as a couple of tubes only 2 1/2 weeks ago.

You might not be aware of this, but during its early development a fetus looks remarkably like something from the dawn of time.

Here, let’s take a closer look. There’s a human fetus’s head during the first month of development, when it was still an embryo. Its face starts as a series of paired tissue mounds called branchial arches.

Let’s take a look from the front. The embryo’s face actually forms from the first branchial arch, along with the area just above it. The forehead and nose form from this area. These areas will form the cheekbones, and these lower areas will form the lower jaw. And this area will form the mouth. At 28 days of development, you can see the lower jaw, which has fused together from the branchial arches. The thickenings you see here will eventually form the nostrils.

By day 31, you can see the nostrils have started to form. And, quite remarkably, the eyes have now appeared on each side of the head.

Two days later, the nostrils have moved toward the center of the face. You can also see that as the ears begin to form, they are positioned in a pretty odd location. But don’t worry, they will move.

At 35 days, the nostrils are even closer together, and we can see more of the eyes. At 40 days, the baby has developed eyelids, and the nose looks much more developed.

Here he is at 48 days and he’s looking pretty darn good. The nasal swellings have joined in the center of the face, and the eyes have moved to the front of the head.

Three weeks later, the fetus looks more human than ever. After that, its face continues to develop more typical proportions right up until the time of its birth.

Let’s look at the entire process again...

As you can see, the development of the face is a fascinating process that has some very dramatic changes taking place in a relatively short amount of time.

The most critical stage of development for the embryo’s nervous system is the third and fourth weeks of pregnancy. Starting with the uterus, let’s enlarge the area where the embryo has implanted. Here’s the embryo implanted in the uterus on day 14. You can see that it lies within the wall of the uterus, and is covered by a single layer of cells.

Take a moment to get oriented. There’s the yolk sac, which makes blood cells for the embryo, and the amnion, which surrounds and protects the embryo, and the blood vessels that will help form the placenta. Also notice that the embryo is connected to the uterus by a small connecting stalk, which will eventually become the umbilical cord.

If we rotate it to the left we’ll get a better view of the back of the embryo, where we can see the brain and spinal cord develop.

On day 14, the embryo looks like a little disc. The first part of the nervous system that forms is an indentation called the neural groove. Over the next seven days, the groove deepens as the cells around it form ridges called neural folds.

By day 27 we see that the neural folds wrap around the neural groove and form the neural tube. The neural fold will become the spinal cord. Those bundles of cells that look like building blocks are called somites. They form the vertebral column, or backbone. They also help form the ribs, and the muscles of the neck, arms, and legs.

Now let’s watch it again without interruption…

If we rotate the embryo again to the left, we can see the areas that will eventually become the brain and spinal cord. The embryo’s nervous system is particularly vulnerable at this stage of development, so an expectant mother should be careful about avoiding any substances that could potentially harm it.

Many people have mistaken ideas about how a growing embryo eats and breathes in the uterus.

From the earliest stages of its development, the growing embryo requires nutrition and oxygen, and a disposal system for the waste products of its own metabolism. All of this is accomplished by the placenta, which allows the growing embryo to eat and breathe while in the mother's uterus.

To get some perspective on how the placenta began, let's go back to Day 8. This hollow ball of cells moving through the uterus is the blastocyst, searching for an implantation site. Here you see its outer layer beginning to extend out and implant in the uterine lining, searching for the uterine blood vessels that would nourish it throughout the pregnancy.

As it went deeper, a single layer of cells from the mother's uterine lining surrounded it, so that it would be protected from harm. On Day 9, as it grew larger and more complex, the blastocyst became an embryo. Here it's about the size of a pinhead.

Also on Day 9, the outer layer of the embryo developed spaces called lacunae. The lacunae filled up with blood from the mother's uterine lining.

On Day 13, small projections from the embryo's chorionic layer reached out into the uterine lining. The chorionic layer is one of the membranes that surround the embryo and help it implant.

On Days 15 through 21, blood vessels began to form beneath this chorionic layer.

Around Day 21, the embryo's blood stream and the mother's blood stream were in such close contact that nutrients and oxygen could cross from mother to embryo. This was how the embryo first got its food and air from the mother, and technically this is when the placenta began to function.

Let's magnify this area so you can see what we're talking about. Here you see a vein and an artery from the embryo in close contact with the blood in the mother's uterine lining. Inside the blood vessels, you can also see red blood cells, which carry oxygen.

The two blood streams are separated by a thin collection of tissues in the placenta called the blood barrier. This barrier permits small particles like nutrients and oxygen to pass from the mother to the embryo, and allows waste products to pass from the embryo back to the mother. The blood barrier also prevents many large or potentially harmful particles from entering the embryo's blood stream. Notice that the red blood cells do not cross from the mother's blood stream to the embryo's.

You may be wondering how a mother's blood cells could be harmful to her growing baby, and why it's important to keep the two blood streams separate. If the mother's blood type is RH negative, and her embryo's blood type is RH positive, then the mother's antibodies would treat the embryo as an invading foreign organism, and try to destroy it.

Now you can see why the placenta and its blood barrier are important for supplying the growing embryo with nutrition and oxygen, removing its waste products, and preventing harmful substances from getting into its blood stream.

A baby’s sex is determined at the time of conception. When the baby is conceived, a chromosome from the sperm cell, either X or Y, fuses with the X chromosome in the egg cell, determining whether the baby will be female or male. Two X’s means the baby will be a girl, and XY means it will be a boy.

But even though gender is determined at conception, the fetus doesn’t develop its external sexual organs until the fourth month of pregnancy.

Let’s go to seven weeks after conception. You can see from the front that the fetus appears to be sexually indifferent, looking neither like a male or a female.

Over the next five weeks, the fetus begins producing hormones that cause its sex organs to grow into either male or female organs. This process is called sexual differentiation.

We don’t know what sex this fetus is yet, so we’ll have to be hypothetical here....Now, if the fetus is a male, it will produce hormones called androgens, which will cause his sexual organs to form like this.

On the other hand, a female fetus would not produce androgens; she would produce estrogens… so her sex organs would form like this.

Now let’s take a look at something you may have missed. At seven weeks, the sex organs of a male and female look identical. Let’s add some color to see what happens during sexual differentiation. Keep your eye on the genital tubercle.

See that? The genital tubercle formed the penis in the male, and the clitoris in the female.

The penis and clitoris are called sexual analogs because they originate from the same structure.

A baby’s skeleton begins as fragile membranes and cartilage, but after three months, the membranes and cartilage start turning into bone, providing protection for the internal organs, and a solid framework for the muscles.

Late in the second month of fetal development, a fetus’ skeleton is made up of thin membranes, which are about the thickness of paper tissue, and soft, flexible cartilage, like the kind you find in your ear. Over time, both types of tissue will turn into bone in a process called ossification.

Ossification occurs in two ways...the first is when membranes turn into bone.

If we look at a fetus during the third month, we can see that the membranes on the side and back of the skull are starting to ossify. That means that the bone tissue is slowly growing over the area where the membranes once existed. Eventually, these bone plates will grow together forming the cranial cavity which protects the brain.

As the baby’s development is close to birth, you can see the bones of the skull still have gaps between them. These gaps, called fontanelles, allow room for the baby’s brain to grow, and also enable the head to be compressed during delivery.

The fontanelles will remain open until the end of the second year. And even though they’re commonly known as the baby’s soft spot, the fontanelles are actually about the thickness and strength of a piece of canvas. Which kind of makes them a soft, but tough, spot.

The bones of the skull won’t stop growing until a child reaches adulthood. That’s when the joints between the bones, called the sutures, will fuse together.

Now let’s go back once again and watch the second type of ossification when cartilage turns into bone. This time we’ll look at the hand. Most of the bones of the skeleton, like the arms, legs, ribs, fingers, and backbone, start off as cartilage.

We can get a good idea of how cartilage turns into bone by looking at this portion of the hand. Here’s what it looks like on the inside.

From the second month until the end of the third month, remarkable changes take place. Watch the middle of the cartilage: both the inside and the outside turn into bone, or ossifies.

This is how the bones will continue to grow until adulthood---from the middle of the bone outward. That way they can continue to increase in their length and width.