ONTOGENY 253 



in the embryonic or adult stages of all chordates. In some of 

 them such as Cephalochorda and some vertebrates (Cyclostomata) 

 it persists in the adult animal. In most vertebrates, however, 

 including the frog, the embryonic notochord is replaced by the 

 centra of the vertebra, developed from tissue derived from the 

 somites. As pointed out in an earlier chapter, in many fishes the 

 notochord persists in the adult in spaces between the ends of 

 vertebrae and to a slighter degree in the axial portions of the 

 centra. In cyclostomes it persists in the adult in a fully devel- 

 oped condition as a long, flexible, tough rod, tapered at the ends, 

 extending from about the middle of the brain to the opposite 

 end of the body and lying directly below the central nervous 

 system. It functions primarily as a supporting structure, is 

 nonnervous in character, and should not be confused with the 

 neural tube (Fig. 155). 



Alimentary Canal. — The cells lining the archenteron after the 

 separation of the notochord from its roof, represents the definitive 

 endoderm of the frog and later forms the lining or mucosa of the 

 alimentary canal, except that of the mouth and of the cloaca. 

 Since the lungs, liver, and pancreas, as well as the thyroid and 

 thymus glands, originate as outgrowths of the embryonic ali- 

 mentary canal, all of these structures are endodermal in origin. 

 The remaining parts of the wall of the alimentary canal, viz., 

 submucosa, muscularis, and serous membrane (visceral peri- 

 toneum) are derived from the lateral unsegmented mesoderm 

 that grows down between the endoderm and ectoderm. About 

 two weeks after fertilization, the tadpole hatches, i.e., it leaves 

 the egg capsule and swims about. By this time the animal has 

 an elongated form with a short tail but no mouth or anal opening. 

 On the underside of the head is a curved sucker, by means of 

 which the tadpole attaches itself to solid objects. The mouth is 

 formed by an invagination of the ectoderm just in front of the 

 sucker to form a pit which deepens until it touches and fuses 

 with the endoderm; the fused layers shortly after become per- 

 forate. An anal opening is formed in a similar way at the pos- 

 terior end of the body. With the formation of the mouth, the 

 sucker begins to atrophy and eventually disappears. Until a 

 mouth is formed the embryo is dependent for nutrition upon yolk 

 stored in the large endodermal cells, particularly those forming 

 the floor of the alimentary canal. After the mouth is formed, the 



