280 



THE GASTRULA OF AMPHIBIA. 



of the medullary folds commences behind, so that the external 

 opening of the blastopore 

 is obliterated simultane- 

 ously with the commencing 7rl / 



formation of the medullary 

 canal ; but in the Frog the 

 closure of the medullary 

 folds commences anteriorly 

 and proceeds 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 disappear. 



The two other types classed above with the Amphibia, viz. 

 Petromyzon and Acipenser, agree sufficiently closely with them 



FIG. 171. LONGITUDINAL VERTICAL SEC- 

 TION THROUGH AN EMBRYO OF PETROMYZON 

 OF 136 HOURS. 



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

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

 cavity. 



FIG. 172. LONGITUDINAL SECTION THROUGH AN ADVANCED EMBRYO OF 

 BOMBINATOR. (After Gotte.) 



;//. mouth ; an. anus ; /. liver ; ne. neurenteric canal ; me. medullary canal ; 

 ch. notochord ; pn. pineal gland. 



to require no special mention ; but with reference to both types 

 it may be pointed out that the ovum contains relatively more 

 food-yolk than that of the Amphibian type just described, and 



