362 THE CHANGING GENERATIONS 



change into adults without radical alteration; these are all coelenterates (Hydra, 

 corals, jellyfishes, etc.)- In them the blastopore becomes the mouth, the archen- 

 teron the gastro vascular cavity, and the body is two-layered (diploblastic). 



Each embryo in the somewhat diminished company that remains now develops 

 a third cell-layer (mesoderm) between the ectoderm and entoderm, thus becoming 

 triploblastic. A few (comb jellies and flatworms) drop out of the procession at this 

 point and become triploblastic adults with a gastrovascular cavity and a mouth 

 but no anus. All the rest continue their development, but they begin to follow 

 divergent paths. For our purposes we need trace only two companies of embryos — 

 those following the chordate path, and those following the annelid-arthropod 

 route. 



In the embryos of each of these groups a new body cavity, the coelom, forms 

 within the mesoderm; the body elongates and becomes subdivided into a linear 

 series of repeated segments, or somites; and a central digestive tube, with anterior 

 mouth and posterior anus is formed. In all other respects development follows 

 quite different lines in the two assemblages. 



In the annelid-arthropod group of embryos a solid, paired ventral nerve cord 

 and a tubular circulatory system appear. The annelid and arthropod paths 

 now diverge. In the annelids the coelom remains large, the blood vessels continue 

 to be tubular, many somites are formed, and each somite typically gives rise to a 

 pair of fleshy, jointless appendages. Further differentiation among these embryos 

 results in the formation of adults of all the varied species of annelid worms. In 

 the arthropods some of the blood vessels swell and fuse to form a huge blood 

 cavity that surrounds the digestive tract and almost crowds out the coelom. A 

 dorsal blood vessel is left to form the dorsal heart and aorta. A semirigid external 

 skeleton forms from the skin, and some or all of the somites develop paired jointed 

 appendages. Further differentiation separates successively the major and minor 

 groups and finally the species of crustaceans, arachnids, insects, and so on. 



In all the chordate embryos (Fig. 15.16) a tubular nerve cord forms along the 

 dorsal side, with a brain at the anterior end. The heart develops ventrally; the 

 somites become intimately fused; a stiff rod of cartilage, the notochord, appears 

 beneath the nerve cord; and a matched series of pharyngeal gill pouches and 

 external gill clefts is formed in the neck region. These things happen whether the 

 embryos are those of fishes, frogs, snakes, chicks, pigs, or men. 



Some of the embryos become adults at about this stage, and never develop a 

 backbone. They are wormlike or vase-shaped marine creatures, whose kinship 

 to the vertebrates might not have been recognized had it not been for the homolo- 

 gous resemblances between their embryos and those of higher chordates. The 

 great majority of the chordates are vertebrates, and their embryos undergo 

 further development. The notochord is replaced by a series of bony structures 

 that become the vertebrae of the backbone, and eyes, ears, paired appendages, and 

 other structures are added. By further changes the division of the vertebrates 

 into fishes, amphibians, reptiles, birds, and mammals begins to appear, and the 

 embryos of each group follow their own paths of development. Fish embryos 

 have a shorter series of changes to undergo before arriving at the adult condition 

 than do those of amphibians, which must pass through most of the stages shown 

 by the fish before they develop their special amphibian characteristics. Similarly 



