140 DEVELOPMENT OF THE FllOG'S EGG [Ch. XII] 



The main differences that exist between the methods of forma- 

 tion of neurenteric canal and anus in the frog and in urodeles 

 are these: In the frog the ventral lip of the blastopore grows 

 forward during the closure of the blastopore, and only subse- 

 quently a new opening forms at the point from which the for- 



Fig. 38. — Embryo of Rana temporaria at time of hatching. 



ward growth began (Fig. 37, A, B). In the urodeles (newt 

 and Am bly stoma) the ventral lip of the blastopore remains 

 stationary, i.e. it retains its first position, and the anus forms 

 directly from its posterior end. 



The Digestive Tract and the ^ Gill-slits 



The origin of the archenteron has been described in Chapter 

 VI. At the time when the yolk-plug is drawn in from the 

 surface, the archenteron has begun to enlarge (Fig. 26, A). 

 A series of cross-sections (Fig. 26, B-E) of an embryo at this 

 stage show that the dorsal and lateral walls of the archenteron 

 consist of a single layer of endodermal cells, while the floor of 

 the archenteron is formed by the upper surface of the yolk- 

 mass. The uppermost cells of the yolk-mass show, to some 

 extent, a tendency to arrange themselves in \ single layer 

 bounding the archenteron. • . 



Shortly after this period the embryo increases in length, and 

 the archenteron is correspondingly drawn out (Fig. 37). The 

 anterior end of the archenteron enlarges, and the yolk-mass is 

 pushed posteriorly. As a result the middle and posterior parts 

 of the archenteric cavity become smaller than they were in the 

 earlier stages (Figs. 39, 40). The walls of the anterior portion 

 of the archenteron are thin, and composed of a single layer of 

 cells. A blind diverticulum extending from this enlarged 



