SEXUAL REPRODUCTION AND DEVELOPMENT OF INDIVIDUAL 217 



are thirty-two somites and the body form is becoming defined. 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 identi- 

 fied definitely as human on the basis of morphological features. 



The younger embryos of different groups of vertebrate animals are 

 so similar that it is impossible to distinguish them from each other. 

 The illustrations in Fig. 203 in a later chapter will bear this out. 

 At a stage early enough, the embryos of the human being, the pig, 

 the rat, the rabbit, the alligator, the salamander, and the fish all ap- 

 pear very similar. The gill slits and segments are conspicuous on all 

 of them. Much later, the limbs develop from lateral limb buds in 

 the mesoderm. The hind limb buds develop first, and the front ones 



follow. 



Organs and Systems 



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

 have been established in the embryo, the next step is the differentia- 

 tion of these layers each in various ways for the formation of par- 

 ticular organs and systems of organs. The development of the 

 typical vertebrate systems is generally similar in all of the different 

 classes of this group. The development of the different organs in 

 the body is often referred to as organogenesis. 



Digestive System. — This system takes its beginning in the arcJien- 

 teron which is formed as a new cavity at the time the endoderm be- 

 comes differentiated from the germ plate of cells. The endoderm 

 becomes the lining of the alimentary canal and its outgrowths, with 

 exception of the mouth and anal cavity. And the splanchnic portion 

 (medial sheet) of the mesoderm comes in to cover it. The muscles, 

 connective tissue, and blood vessels in the wall of the canal are pro- 

 vided by this mesoderm sheet. Immediately posterior to the mouth 

 is the pharynx, which has lateral gill pouches or pharyngeal pouches. 

 In the higher vertebrates there are typically five pairs of these 

 pouches. In fishes and amphibians these pouches break through to 

 the exterior, forming gill clefts. In higher forms these pouches are 

 transient structures which remain to call attention to existence of 

 .functional gills in their phylogenetic ancestors. Certain adult struc- 

 tures do arise from these pouches even in mammals. Such structures 

 as the middle ear. Eustachian tube, hyoid, thyroid, thymus, tonsil, 

 and others are products of these pouches. 



The tubular canal is inflated and curved in the formation of the 

 stomach some distance posterior to the pharynx. The small and the 



