2 9 



THE EVOLUTION OF MAN. 



albuminous stratum, which has been externally deposited. 

 In future we shall call this membrane the outer egg-mem- 

 brane, the primary chorion (prochorion, a). The real wall 

 of the vesicle, surrounded by this outer egg-membrane, 

 consists of a simple layer of exoderm-cells (6), which have 

 been regularly flattened by mutual pressure, and most of 

 which are hexagonal ; a light-coloured kernel is visible 

 through their finely granulated protoplasm (Fig. 74). On a 



FIG. 74. Four exoderm-cells from the intestinal germ-vesicle of a 

 rabbit. 



FIG. 75. Two entoderm-cells from the same. 



point on the inside of this hollow sphere lies a circular disc, 

 formed of darker, softer, and rounder cells, of the dark 

 granulated entoderm-cells (Fig. 75). 



The characteristic germ-form in which the developing 

 Mammal now is has usually been called the " germ-vesicle " 

 (Keimblase, Bischoff) ; the " sac-germ " (Baer) ; the " vesi- 

 cular embryo," or the " germ-membrane vesicle " (vesicula 

 blastodermica, or, briefly, blastosphcera}. The wall of the 

 hollow sphere, consisting of a single cell-stratum, was called 

 the " germ-membrane," or blastoderm, and it was supposed 

 to be equivalent to the cell-stratum, called by the same 

 name, which forms the wall of the true germ-membrane 

 vcsiclo (Blastula) of the Amphioxus (Plate II. Fig. 4), and 



