454 



OF GENERATION AND DEVELOPMENT. 



dary envelope, constituting the germinal membrane ; and as this forms 

 a complete sac enveloping the liquefied yolk of the interior, and as the 



Fig. 139. 



Latter stage in the segmentation of the yolk of the Mammalian Ovum ; at A is shown the " mulberry 

 mass" formed by the minute subdivision of the vitelline spheres ; at B, a further increase has brought its 

 surface into contact with the vitelline membrane, against which the spherules are flattened. 



whole structure of the future embryo originates in its substance, it has 

 been termed by Bischoff the blastodermic vesicle. 



806. The Blastodermic Vesicle very soon after its formation, pre- 

 sents at one point an opaque roundish spot, which is produced by an 

 accumulation of cells and nuclei of less transparency than elsewhere ; 

 and it is within this, which is termed the area germinativa, that all the 

 structures of the permanent organism originate. When seen in section, 

 this mass of cells presents the aspect shown at Fig. 140, c. The Ger- 

 minal Membrane increases in extent and thickness by the production 

 of new cells ; and it subdivides into two. layers, which, although both at 

 first composed of cells, soon present distinctive characters, and are con- 

 cerned in very different ulterior operations. The outer one of these is 

 commonly known as the serous layer, and the inner as the mucous ; 

 the division is at first most evident in the neighbourhood of the area 

 germinativa ; but it soon extends from this point, and implicates nearly 

 the whole of the germinal membrane. 



807. The area germinativa soon loses the rounded form which it at 

 first possessed, and becomes first oval, and then pear-shaped. Whilst 



Plan of early Uterine Ovum. Within the external ring, or zona pellucida, are the blastodermic 

 vesicle, a; the yolk, b; and the incipient embryo c. 



this change is taking place in it, there gradually appears in its centre 

 a clear space, termed the area pellucida ; and this is bounded externally 



