122 



FORMATION OF THE LAYERS. 



attention, at the end of the section, to the points in which the 

 Newt is peculiar. 



The segmentation of the Frog's ovum has already been 

 described (Vol. II. pp. 95-7), but I may remind the reader that the 

 segmentation (fig. 69) results in the formation of a vesicle, the 

 cavity of which is situated excentrically; the roof of the cavity 

 being much thinner than the floor. The cavity is the segmenta- 

 tion cavity. The roof is formed of two or three layers of smallish 

 pigmented cells, and the floor of large cells, which form the 



FIG. 69. SEGMENTATION OF COMMON FROG. RANA TEMPORARIA. 



(After Ecker.) 



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



greater part of the ovum. 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 increase in 

 number, and those of the roof become divided into two distinct 

 strata (fig. 70, cp\ 



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 consti- 

 tuted of epiblast. 



The next changes which take place lead (i) to the formation 



