196 



EMBRYOLOGY. 



obstacle which it encounters in the presence of the yolk and acquires 

 its ultimate 



In the second and likewise more extensive part we must concern 

 ourselves more minutely with the embryonic enveloping structures 

 and appended organs, which subserve various purposes. 



The collection of yolk-material disturbs the course of development 

 least in the case of the Amphibia. The latter therefore stand, 

 as it were, midway between Amphioxus with direct development 



and the remaining Verte- 

 brates, and constitute a 

 transition between them. 

 In the Amphibia the yolk 

 shares in the process of 



Fig. 118. Diagrammatic longitudinal section through 

 the embryo of a Frog, after GOETTE, from BALFOUR. 



nc, Neural tube ; x, communication of the same with 

 blastopore and coelenteron (al) ; yk, yolk-cells ; m, 

 middle germ-layer. For the sake of simplicity the 

 outer germ-layer is represented as if composed of 

 a single layer of cells. 



cleavage ; after the close of 

 this process it is found ac- 

 cumulated for the most part 

 in the large yolk-cells which 

 form the floor of the blastula 

 (fig. 45) ; at the time of the 

 differentiation into gerrn- 

 layers it is taken up into the 

 coelenteron, which it almost 

 completely fills (fig. 47); after 

 the formation of the body- 

 sacs the large yolk-cells lie 

 in a similar manner in the- 



ventral wall of the intestine proper (fig. 118 yk). Here they are in 

 part dissolved and employed for the growth of the remaining parts 

 of the body, in part they share directly in the formation of the 

 epithelium of the ventral wall of the intestine. 



In consequence of the presence of the great accumulation of yolk- 

 cells, the Amphibian embryo acquires a shapeless condition at a time 

 when the Amphioxus larva has already become elongated and fish- 

 like. The body, which is spherical during gastrulation, later becomes 

 egg-shaped, owing to its elongation. Thereupon the head-end and 

 the tail-end begin to be established at the two poles as small eleva- 

 tions (figs. 118 and 80). The middle or trunk-part lying between 

 the latter becomes somewhat incurved along its dorsal region, in 



