AMPHIBIA. 



101 



the tioor of large cells, which form the greater part of the ovum. 



Fig. 69. Segmentation of Common Frog. Eana Tempokaria. (After Ecker. ) 

 The numbers above the figures refer to the number of segments at the stage figured. 



These large cells, which are part of the primitive hypoblast, will be 

 spoken of in the sequel as yolk-cells : they are equivalent to the 

 food -yolk of the majority of vertebrate ova. 



The cells forming the roof of the cavity pass without any sharp 

 boundary into the yolk-cells, there being at the junction of the two 

 a number of cells of an intermediate character. The cells both of the 

 roof and the floor continue to in- 

 crease in number, and those of 

 the roof become divided into two 

 distinct strata (fig. 70, ep). 



The upper of these is formed 

 of a single row of somewhat cubical 

 cells, and the lower of several rows 

 of more rounded cells. Both of 

 these strata eventually become the 

 epiblast, of which they form the 

 epidermic and nervous layers. The 

 roof of the segmentation cavity 

 appears therefore to be entirely 

 constituted of epiblast. 



The next changes which take 

 place lead (1) to the formation 

 of the mesenteron', and (2) to the 

 enclosure of the yolk-cells by the 

 epiblast. 



The mesenteron is formed as in 

 Petromyzon and Lepidosteus by 

 an unsymmetrical form of invagination. The invagination first com- 

 mences by an inflection of the epiblast-cells for a small arc on the 



1 Since the body-cavity is not developed as diverticula from the cavity of invagina- 

 tion, the latter cavity may conveniently be called the mesenteron and not the archeu- 

 teron. 



Fig. 70. Section through Frog's 

 ovum at the close of segmentation. 

 (After Gotte.) 



sg. segmentation cavity; II. large yolk- 

 containing cells; ep. small cells at forma- 

 tive pole (epiblast) ; X. point of inflection 

 of epiblast; y. small cells close to junction 

 of the epiblast and yolk. 



