412 DEVELOPMENT OF FROG AND OTHER VERTEBRATA 



Comparison of longitudinal and transverse sections (Figs. 215 E 

 and 216 B) at a stage when the neural folds have just completed 

 their fusion shows that the general topography of the vertebrate 

 animal is being rapidly established. At a shghtly later stage (Figs. 

 216 C and 217), when the coelome has developed as a cleft in the 

 mass of mesoderm cells on either side of the enteron and yolk-laden 

 cells, the muscle segments have been differentiated from the meso- 

 derm on either side of the notochord, and the resemblance to a trans- 



FiG. 216. — Successive stages in the development of the general body plan of 

 the frog as shown by transverse sections. A, early neural-fold stage (c/. 

 Fig. 215 A). B, stage in which neural folds have recently closed to form the 

 neural tube (cf. Fig. 215 C). C, stage in which the ca4ome and other features 

 of the body plan are apparent (cf. Figs. 215 E and 217). 



c, notochord; cop, coelome; e, archenteron; ec, ectoderm; en, endoderm; I, liver; m, 

 mesoderm; m.s., mesodermal segment; n, neural canal; n.c, neural crest; ng., neural 

 groove; n.p., neural plate; n.t., neural tube; prn, pronephros; j/.c, yolk-laden cells. 



verse section of the adult frog (Fig. 19, p. 40) becomes apparent. 

 The dorsal tubular nervous system, the axial skeleton now repre- 

 sented by the notochord and later by the vertebral column, the 

 digestive cavity lined with endoderm, the visceral and parietal 

 regions of the mesoderm with the coelome between them, and the 

 ectodermal covering of the body indicate the fundamental chor- 

 date organization in a manner that is unmistakable. As shown 

 by the outlines of these stages (Fig. 215), the antero-posterior 

 elongation of the embryo begins during the neural-fold stage and 

 thus the head and tail regions of the tadpole make their appearance. 

 The Tadpole Stages. — The elongation of the embryo during 

 the neural-fold stage is coincident with the internal development 

 of the general chordate organization. The conversion of the late 



