THE PROCHORION. 289 



re-discovered by Baer ; those of the Rabbit were afterwards 

 more minutely described by Bischoff, in 1842. They may 

 be found in the uterus (matrix) of the Rabbit, the Dog, and 

 other small Mammals within a few days after impregnation. 

 The ripe mammalian eggs, having left the ovary, are fer- 

 tilized, either here or in the oviduct, by the active sperm- 

 cells which make their way in. 92 (On the uterus and 

 oviduct cf. Chapter XXV.) Cleavage and gastrulation 

 take place within the oviduct. Either while the mam- 

 malian Gastrula is still in the oviduct, or after it has entered 

 the uterus, it changes into the globular vesicle which is 

 represented in Fig. 72 • (the surface) and in Fig. 73 (in 



Fig. 72. — Intestinal germ-vesicle (Gastrocystis) of a Eabbit (the so-called 

 "Germ-vesicle," or vesicula blastodermica, of other writers): a, external 

 egg-membrane (chorion) ; b, skin-layer (exoderma), forming the whole wall of 

 the germ-yelk vesicle ; c, heap of dark cells, forming the intestinal layer 

 (entoderma) . 



Fig. 73.— The same in section. The letters as in Fig. 72 : d, hollow 

 space within the intestinal germ-vesicle 



section). The thick, external, structureless membrane which 

 surrounds this is a modification of the original egg-mem- 

 brane (zona pellucida, p. 135), with the addition of an 



