COMPARISON OF THE GERMINAL LAYE11S. 



233 



small difference between the Frog and Amphioxus 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 

 commences behind, so that the 

 external opening of the blas- 

 topore is obliterated simul- 

 taneously with the commenc- y> 

 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 Fl - m - LONGITUDINAL VERTICAL SECTION 



i T. r ,1 , THROUGH AN EMBRYO OF PETROMYZON OF 136 



obliteration or the external HOURS. 



opening of the blastopore is a me . mes oblast ; y k. yolk-cells; /. alimen- 



late event in the formation of tary tract; bl. blastopore; s.c. segmentation 



the medullar canal. 



cavity. 



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 sufficiently close- 

 ly with them to require 

 no special mention ; but 

 with reference to both 



. , i AVJ . J_ | J J_JW11 kJ J. J. \.< -fA.-^ .-iJJ Oi^VyXi.W1 A JJ.J.H./ \.i \J JJ 



types it may be pointed VANCED EMBRYO OF BOMBINATOR. (After Gotte.) 

 out that the OVUm con- m. mouth; an. anus; I. liver; ne. neurenteric canal ; 



tains relatively more me. medullary canal; eft. notochord; #w. pineal gland, 

 food-yolk than that of 



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

 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 Elasmobranchii. 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. I7'2 LONGITUDINAL SECTION THROUGH AN AD- 



