214 THE EVOLUTION OF MAN. 



cess in which the inner intestinal layer cells are overgrown, 

 is really nothing but the inversion of the vegetative hemi- 

 sphere into the animal hemisphere of the germ-vesicle ; i.e. 

 the formation of a Gastrula (Fig. 41).^^ 



Next, therefore, follows a stage in which the mamma- 

 lian germ consists of 12 cleavage-cells; 4 darker entoderm- 

 cells form a three-sided pyramid which is covered by a cap 

 of 12 lighter exoderm-cells (Plate II. Fig. 15 in section). 

 The next stage, in which there are 16 cleavage-cells, is seen 

 to consist of 4 entoderm-cells in the interior, 4 other outer 

 and lower entoderm-cells ; while the 8 exoderm-cells, in the 

 form of a hemispherical cap, cover the upper half of the 

 germ. This cap of exoderm-cells, which increase in number 

 from 8 to 16, continues to overgrow the inner cell mass ; of 

 the 8 entoderm-ceUs, 3, 4, or 5 lie in the centre of the germ, 

 and the rest at the base of the globular germ (Plate II. 

 Fig. 16). This 24-celled stage is followed by one in which 

 there are 32, for the 8 entoderm-cells also double their 

 number. Tliis is afterwards succeeded by germ-forms in 

 which there are 48 cleavage-cells (32 exoderm and 16 ento- 

 derm) ; 64 cleavage-cells (32 skin-layer and 32 intestinal 

 layer) ; 96 cleavage-cells (64 exoderm and 82 entoderm), 

 and so on. 



When the mammalian embryo has acquired 96 cleavage- 

 cells, a stage which, in the case of the Eabbit, is reached 

 in about the 70th hour after fertilization, the charac- 

 teristic form of the Hood-gastrula (Amphigastrula) becomes 

 plainly visible (Fig. 41 ; cf Plate II. Fig. 17 in section). 

 The globular embryo consists of a central mass of 32 soft, 

 roundish, dark granular entoderm-cells, which, by mutual 

 pressure, are flattened multilaterally, and which assume 



