Fetal Anatomy--Starting at the top
The Fetal Brain and the Skull
The ultimate transformation of a fertilized blob of genetically directed, chemically driven cells into the very person reading this--you--is an amazing process. It all starts off, of course, with the eggs your mother was born with. (You are really as old as your age + the age of your mother when she conceived, because that is how old her eggs were at the time.) With fertilization, as mentioned above, the genetics direct and the chemicals drive the cell to split over and over according to a certain architecture. Eventually there are arms and legs and eyes, a stomach, liver, and everything else, hopefully where they all belong. Of course, one structure, the brain, makes us what we are on the food chain. The superior brain of Homo sapiens, with the help of an opposable thumb, has allowed us to dominate and run the world. (Whether our brains are superior enough to do that well is a whole other topic!)
The brain of the developing fetus changes so much over the course of gestation that it is even possible to date the stage of the pregnancy by what the brain looks like. What becomes progressively more pronounced and what makes us the thinking beings that we are is the development of neurons (nerve cells). Nature has taken advantage of a property of geometry to cram in as many neurons as possible into a relatively small space--that property being surface area.
To illustrate by way of example, if you were to drive the relatively straight interstate from Los Angeles to San Francisco, you would travel a certain distance that would be recorded on your car's odometer. But if you were to take Highway 1, which follows the curves, bulges, inlets, and other miscellaneous variations of the coast, the odometer is going to record a larger number. Now I know we're talking about a whole lot of tennis balls either way, but the theoretical number of tennis balls that could be lined up along curvy Highway 1 will dwarf the theoretical number that can be lined up along the interstate. The mathemeticians will glibly point out that this is fractal, or "non-Euclidean" geometry. But our DNA thought of it before it was ever described in a math book, and this is the way we can cram into our brains so much more thinking and reasoning power. For the brain is not just a chunk of connected nerve cells, but bundles and tracts that interweave--up and down, over and under, in and out--presenting as the famous convolutions that is the crowning glory of our species.
If you were to look at a human brain, you'll see these convolutions, serpentine, thick tissue cylinders. If you were to measure along these structures, you would mark real estate along the bulges and into the valleys (called "sulci") between them. The more convoluted the brain is and the deeper the sulci, the more surface area there is to pack extra neurons. It is during the second trimester (around 20 weeks) that the sulci begin developing, and the quantum leaps in further development begin after 28 weeks. By 40 weeks, or term, the brain is a masterpiece of architecture which, because of convolutions, takes the mental abilities of a brain that should be about 10 feet wide and puts them into a package that will fit comfortably inside your typical head.
The Second Point: Fetal Brain
The second aspect I would like to touch upon is how does that head, with its precious masterpiece of a brain, come out of the mother intact? Putting a sphere of bone around it helps, certainly. But the bony capsule cannot be too rigid, or it would have trouble passing through the maternal pelvis at birth. Herein is a dilemma that once again our DNA has solved:
1) Brain needs protection on the way out, and
2) the protection can't keep it from getting out
The skull is not one sealed container, but a collection of plates, attached but mobile, much like the tectonic plates on the Earth. Because there are fault lines between tectonic plates (probably running along Highway 1 I would suspect), the land masses are able to move. On the skull, these faults, or separations, are called by an old name, "sutures," and have nothing to do with the modern sense of stitching. The sutures of the skull run between the bony plates that together make up a helmet of protection for our quarterback, the brain. But because of these sutures, the plates can move--there is flexibility, which is so necessary when trying to cross all of the altering diameters of the maternal pelvis.
The newborn's skull is often first seen as being similar to the shape of his or her mother's internal pelvis. This misshapen head is usually back to it's cute, round shape withing a couple of days, but when particularly severe can raise the eyebrows of the new parents, for sure.
At birth the brain doesn't stop growing, as is true with the rest of the body. Therefore there's considerable slack that the sutures provide so that the skull can balloon out with the expansion of the brain as it grows. Just as the separations among the bones of the skull allow the head to mold itself to a shape that can deliver, they also allow spread for the brain to increase in size further. There are areas where there is considerable distance between the skull plates, where the sutures are widest, and these are called "fontanelles." There are four main fontanelles. The anterior one is the famous "soft spot" of the baby's head. There is also a corresponding one at the back of the head and one on either side (the temporal fontanelles). At birth, the sutures and fontanelles allow the protective skull around the brain to be somewhat flexible as the baby's head negotiates that passageway to the outside world. Because of the way the skull is shaped, the displacement of one skull plate results in a force against the brain that is distributed somewhat evenly all around. Even though the skull is not anatomically an intact ring structure (due to sutures and fontanelles), it is nevertheless functionally a ring structure to protect the brain from the compression/decompression exerted against it by the maternal pelvis, the vaginal walls, and the forces of labor.
It is the single best case of having your cake and eating it, too, from an anatomical standpoint.
After birth, the skull plates continue growing toward each other, ultimately fusing together the skull into a solid bony globe that continues to protect the brain, our most cherished organ (which is pretty unusual for a man to state). Ultimately, the sutures, fontanelles, and flexibility are no longer necessary, because the protection that our skulls provided us at birth have allowed us to be smart enough not to stick our heads into anything that tight again.