MAMMALIAN DEVELOPMENT 817 



ing a neural groove. These folds meet each other over the groove to 

 form the neural tube (Fig. 423). 



The mesoderm along each side of the neural plate becomes or- 

 ganized into blocklike thickenings. These are somites, and they are 

 paired opposite each other, marking segmentation in the body. In 

 a chick embryo of thirty-six hours of incubation there are fourteen 

 pairs of somites and in a pig embryo 6 millimeters long there are 

 thirty-two pairs. 



The younger embryos of different groups of vertebrates are so simi- 

 lar that it is impossible to distinguish them from each other. This is 

 illustrated in Fig. 444 in a later chapter. At a sufficiently early 

 stage, the embryos of the human being, the pig, the rat, the alligator, 

 the salamander, and the fish all appear very similar. The gill slits 

 and segments are conspicuous in all of them. IMuch later the limbs 

 develop from lateral pairs of limb buds in the mesoderm. The hind 

 limbs develop first and the front ones follow. The human body can 

 barely be discerned in embryos of one month, and the embryo must 

 be nearly two months of age before it can be identified definitely as 

 human on the basis of morphological features. 



Organs and Systems 



After the three germ layers (ectoderm, endoderm, and mesoderm) 

 have been established in the embryo, the next step is differentiation 

 of these layers each in various ways for the formation of particular 

 organs and systems of organs. The fate of the germ layers has been 

 concisely summarized in the last paragraph in the chapter on meta- 

 zoan organization. 



Embryonic Membranes. — All three of the terrestrial groups of 

 vertebrates (reptiles, birds, and mammals; i.e., amniota) produce 

 extensive embryonic membranes as a feature of their development 

 which is not found in the aquatic forms. These membranes serve 

 to give the developing embryo added protection, as well as increased 

 facilities for the functions of nutrition, respiration, and excretion. 

 In birds and reptiles the early embryo is flat and the somatopleure 

 extends over the surface of the yolk far beyond the limits of the 

 embryo proper. A fold of this sheet of somatopleure (ectoderm 

 and parietal mesoderm) grows dorsally along each side of the embryo 

 (Fig. 424). These folds finally meet each other above the embryo and 

 fuse together, thus forming an enclosed cavity between the dorsal sur- 



