In the fly, segments fuse and display new features such as wings or legs. Illustration by Alison Schroeer
One of the fundamental features of the organization of multicellular animals is segmentation: We are initially built by subdividing a relatively undifferentiated embryonic tissue into smaller, repeated elements, like a stack of mostly identical building blocks. Look at an earthworm or a caterpillar or a maggot, and the organization is clear, with the wormlike animal showing the obvious seams and subdivisions that constitute its assembly from rings of similar chunks of tissue. Another property of this pattern of organization is that individual segments can then acquire specializations. In a caterpillar, the front end is modified with mouthparts and sense organs to form a head, while other segments will bear stubby limbs or be festooned with bristles or colored spots and patterns. Specialization is carried further when a maggot becomes a fly. Segments become much more obscure, retaining their visible identity in the abdomen, but are otherwise fused, elaborated upon, and display new features, such as wings or legs or mouthparts, that make the segments, ultimately, look very different from one another.
We vertebrates were also overtly segmented animals early in our embryonic development. As with the fly, the nature of our construction from similar blocks of tissue has been obscured by later additions in development, with limbs patched on and some segments (like human tails) reduced to near invisibility. Others (like significant portions of our brains) have been telescoped, contorted, and fused so that the boundaries between the original segments are detectable only to sensitive molecular probes. As with the fly's abdomen, we also retain some still apparent vestiges: the chain of vertebrae in our backs and the muscles of our torsos.
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Before those specializations intruded, however, there was an early period in development when all was simple, and the only job the embryo had to do was to set up partitions, segregate small sections of tissue that were all nearly identical, and let local developmental programs proceed within them. The most obvious expression of the process of segmentation is seen in the mesoderm—the embryonic tissue that will form bone and muscle—which begins as a long strip of cells in a continuous mass stretching the length of the embryo, and ends with the mass clumping into a chain of small segments of mesoderm, called somites. One somite (or a few) forms early, and then another coalesces just behind it, and a little later another one behind that, until the entire chain is constructed sequentially, from the front of the animal to the back. We can even watch this happen. I've put a short time-lapse recording of the process in a zebrafish embryo at http://scienceblogs.com/pharyngula/zebrafish. The movie covers about two hours of the embryo's life, but has been sped up 1200 times so that you can easily see the events. At the beginning, four lozenge-shaped somites are visible on the left, and to the right is a ribbon of tissue. As the movie proceeds, portions of the ribbon are pinched off and added to the stack of new formed somites. The zebrafish continues to pinch off new somites until it reaches a total of 30 to 34. Some animals, such as snakes, may continue to form as many as 400.