196 



EMBRYOLOGY. 



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

 its ultimate form. 



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 

 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 blast ula 

 (fig. 45) ; at the time of the 

 differentiation into germ- 

 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 ?//(). Here they are in 

 part di olved 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- 

 n-lls. the Amphibian embryo acquires a shapeless condition at a time 

 when the Amphioxus larva has already become elongated and \\A\- 

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

 h.-ijied, owniir to its elongation. Thereupon the head-end and 

 the tail end Ix-^in to be established at the two poles as small < ! 



< tij^s. 118 and 80). The middle or trunk-part lying bet\\. n 

 the latter becomes somewhat incurvid along its dorsal region, in 



Fig. 118. Diagrammatic longitudinal section through 

 the embryo of a Frog, after GOETTE, from B/.LFOUR. 



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

 blastopore arid ccelenteron (ai) ; 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. 



