36 WALTER HBAPE. 



a surface view (vide fig. 4) it is seen to be rounded. The oval 

 shape in section is due to it being flattened out, and it is for 

 this reason also that the nuclei of the outer layer appear in a 

 surface view larger than those of the inner mass (fig. 4). 



The inner mass is solid, more or less rounded in form, and 

 is attached on one side to the wall of the vesicle. The cells of 

 which it is made up are always, after treatment with picric acid, 

 closely adherent to one another, and are sharply marked off 

 from the cavity of the vesicle (vide figs. 17 and 18). 



The specimen drawn in fig. 19, however, was treated with 

 silver nitrate and preserved in weak glycerine, afterwards being 

 transferred to spirit, embedded, and cut into sections ; in it the 

 cells are much more loosely held together, and in another spe- 

 cimen I have, which was similarly preserved, the same appearance 

 presents itself. 



The irregularly-rounded cells of the inner mass, which are 

 very considerably smaller than either the cells of the inner 

 mass in the fully-segmented ovum, or of the specimen drawn in 

 fig. 2, are composed of granular protoplasm, and many of their 

 nuclei exhibit the modifications attending cell division. 



As the vesicle continues to enlarge the inner mass also now 

 increases in size, changes its shape, and becomes flattened out 

 along the side where it adjoins the outer layer ; and further, 

 the cells of which it is now composed become differentiated into 

 two layers. 



The differentiation occurs in the following manner : — Certain 

 of the cells bordering the blastodermic cavity become separated 

 off from the main portion of the inner mass, and form a single 

 layer of cells bounding the mass on its inner side. 



This layer is the hypoblast. 



The hypoblast is, therefore, derived from cells which result 

 from the multiplication of the inner cell mass present in the 

 fully-segmented ovum. 



Figs. 20 to 23 adequately represent these changes as they 

 take place ; the cells here and there along the lower border 

 of the inner mass become more flattened than their fellows, 

 and stain more deeply with hasmatoxylin (fig. 20) ; gradually a 



