AMPHIBIA. 



107 



Fig. 75. Tkansvekse section through the 



CEPHALIC REGION OF A YOUNG NeWT EMBRYO. 



(After Scott and Osborn.) 



In.luj. invagiiiaied hypoblast, the dorsal part 

 of which will form the notochord; ep. epiblast 

 of neural plate; sp. splanchnopleure; al. ali- 

 mentary tract; yk. and T/;?/. yolk-cells. 



The hypoblast. There are no important points of difference in 

 the relations of tlie hypoblast between the Anura and Urodela. The 

 mesenteron, at the stage re- 

 presented in fig. 73, forms a Tnyiy 

 wide cavity lined dorsally by 

 a layer of invaginated hypo- 

 blast, and ventrally by the 

 yolk-cells. The hypoblast is 

 continuous laterally ar;d in 

 front with tbe yolk-cells 

 (figs. 72, 74 and 75). At an 

 earlier stage, when the me- 

 senteron has a less definite 

 form, such a continuity be- 

 tween the true hypoblast and 

 the yolk-cells does not exist 

 at the sides of the cavity. 



The definite closing in of 

 the mesenteron by the true 

 hypoblast-cells commences in 

 front and behind, and takes place last of all in the middle (fig. 76). 

 In front this process takes place with the greatest rapidity. The 

 cells of the yolk-floor become continuously differentiated into hypo- 

 blast-cells, and very soon the whole of the front end becomes com- 

 pletely lined by true hypoblastic cells, while the yolk-cells become 

 confined to the floor of the middle part. 



The front portion of the mesentenm gives rise to the oesophagus, 

 stomach and duodenum. Close to its hinder boundary there appears 

 a ventral outgrowth, which is the commencement of the hepatic 

 diverticulum (fig. 76, Z). 



The yolk is thus post- „g 



hepatic, as in Verte- 

 brates generally. 



The stomodoeum is 

 formed comparatively 

 late by an epiblastic 

 invagination (fig. 76, 

 m). 



It sliould be noticed 

 that the convei-sion of 

 the yolk-cells into liypo- 

 bla:<t-cells to form the 

 ventral wall of the an- 

 terior region of the ali- 

 mentary tract is a closely 

 similar occui-rence to the 

 formation of cells in the 

 yolk-floor of the anterior part of the alimentary tract in Elasmobrancliii. 



P". 



Fig. 76. Longitudinal section tukough an ad- 

 vanced EMBRYO OP BoMBiNATOR. (After Gotte.) 



m. mouth ; an. anus ; I. liver ; ne. neurenteric 

 canal; mc. mednllary canal; ch. notochord; pn. pineal 

 gland. 



