AMPHIBIA. 



107 



The hypoblast. There are no important points of difference in 

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

 mesenteron, at the stage re- 

 presented in fig. 73, forms a 

 wide cavity lined dorsally by 

 a layer of invaginated hypo- 

 blast, and ventrally by the 

 yolk-cells. The hypoblast is 

 continuous laterally and in 

 front with the yolk-cells 

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

 earlier stage, when the me- 

 senteron has a less definite 

 form, such a continuity be- Fli; 7 - TBANSVEBSE S , CTION THROimH THE 



tweeil the true hypoblast and CEPHALIC REGION OF- A YOUN;; NEWT EMBRYO. 



the yolk-cells does not exist (After Scott and Osborn.) 



/ * T 1 



at the sides of the cavity. 



v 



The definite closing in of 



In.lnj. invaginated hypoblast, the dorsal part 

 of which will form the notochord; ep. epiblast 

 of neural plate: .^. splanchnopletire; al. ali- 

 the mesenteron by the true meutary tract ; yk. and Yhy. yolk-cells. 



hypoblast-cells commences in 



front and behind, and takes place last of all in the middle (fig. 7ri). 

 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 nK'senuTon 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. 7(5, /). 

 The yolk is thus post- 

 hepatic, as in Verte- 

 brates generally. 



The stomodseum is 

 formed comparatively 

 late by an epiblast ic 

 imagination (fig. 70, 

 111). 



It should be, noticed 

 that the conversion of 

 the yolk-cells into liypo- 

 blast-cells to form the 

 ventral wall of the an- 

 terior region of the ali- 

 mentary tract is a closely 

 similar occurrence to the 

 formation of cells in the 

 yolk-floor oi' the anterior part of the alimentary tract in Elasmobranchii. 



FK;. 76. LONGITUDINAL SECTION THROUGH AN AD- 

 VANCED EMBRYO OF BouBiNATOR. (After Giitte.) 



m. mouth; tin. anus; /. liver; nc. neurenteric 

 canal; inc. medullary canal ; ch. notochord; pn. pineal 

 gland. 



