COMPARISON OF THE GERMINAL LAYERS. 



233 



Fig. 171. Longitudinal vertical section 



THROUGH AN EMBRYO OF PeTROM^ZON OF 136 

 HOURS. 



me. mesoblast; yh. yolk-cells; al. alimen- 

 tary tract; bl. blastopore; s.c. segmentation 

 cavity. 



small difference between the Frog and Ampbioxus in the relation 

 of the neural canal to the blastopore. In both types the medullary 

 folds embrace and meet behind 

 it, so that it comes to oc- 

 cupy a position at the hind 

 extremity of the medullary 

 groove. In Amphioxus the 

 closure of the medullary folds i 

 commences behind, so that the 

 external opening of the blas- 

 topore is obliterated simul- 

 taneously with the commenc- 

 ing formation of the medullary 

 canal ; but in the Frog the 

 closure of the medullary folds 

 commences anteriorly and pro- 

 ceeds backwards, so that the 

 obliteration of the external 

 opening of the blastopore is a 

 late event in the formation of 

 the medullary canal. 



The anus is formed {vide fig. 172) some way in front of the 

 blastopore, and a post- anal gut, continuous with the neurenteric canal, 

 is thus established. 

 Both the post-anal gut 

 and the neurenteric 

 canal eventually dis- 

 appear. 



The two other types 

 classed above with the 

 Amphibia, viz. Petro- 

 myzon and Acipenser, 

 agree sufhcieutly close- 

 ly with them to require 

 no special mention ; but 

 with reference to both 

 types it may be pointed 

 out that the ovum con- 

 tains relatively more 

 food-yolk than that of 



the Amphibian type just described, and that this leads amongst otherfj 

 things to the lower layer cells extending up the sides of the segmen-/) 

 tation cavity, and assisting in forming its roof. 



The next type to be considered is that of Elasniobranchii, The 

 yolk in the ovum of these forms is enormously bulky, and the seg- 

 mentation is in consequence a partial one. At first sight the 

 differences between their development and that of Amphibia would 

 appear to be very great. In order fully to bridge over the gulf 



Fig. 172. Longitudinal section through an ad- 

 vanced EMBRYO of Bombinator. (After Gotte.) 



m. mouth; an. amis; I. liver; ue. neurenteric canal ; 

 mc. medullary canal; ch. notochord; pn. pineal gland. 



