THE EEPRODUCTIYE ORGAXS. 747 



membrane, is a transparent membrane, about -grVo ^ an inch (ID/*) in 

 thickness, which under the microscopic appears as a bright ring (fig. 

 445), bounded externally and internally by a dark outline. Within this 

 transparent investment or zona pellucida, and usually in close contact 

 with it, lies the yolk or vitellus, which is composed of granules and glob- 

 ules of various sizes, imbedded in a more or less fluid substance. The 

 smaller granules, which are the more numerous, resemble in their appear- 

 ance, as well as their constant motion, pigment-granules. The larger 

 granules or globules, which have the aspect of fat-globules, are in 

 greatest number at the periphery of the yolk. The number of the gran- 

 ules is greatest in the ova of carnivorous animals. In the human ovum 

 their quantity is comparatively small. 



In the substance of the yolk is imbedded the germinal vesicle, or ves- 

 icula germinativa, -5^- of an inch (.05 mm.) (fig. 445). The vesicle is of 

 greatest relative size in the smallest ova, and is in them surrounded 

 closely by the yolk, nearly in the centre of which it lies. During the 

 development of the ovum, the germinal vesicle increases in size much less 

 rapidly than the yolk, and comes to be placed near to its surface. It 

 consists of a fine, transparent, structureless membrane, containing a clear, 

 watery fluid, in which are sometimes a few granules; and at that part of 

 the periphery of the germinal vesicle which is nearest to the periphery of 

 the yolk is situated the germinal spot, or macula germinativa, of a finely 



Nucleus or germinal vesicle. 

 Nucleolus or germinal spot. 



-Space left by retraction of 

 yolk. 



Yolk or vitellus. 



23^ Vitelline membrane. 



Fig. 445. Semidiagrammatic represention of a human ovum, showing the parts of an animal 



cell. (Cadiat.) 



granulated appearance and of a yellowish color, strongly refracting the 

 rays of light. 



Such are the parts of which the Graafian follicle and its contents, 

 including the ovum, are composed. With regard to the mode and order 

 of development of these parts there is considerable uncertainty. 



The Graafian follicles are formed in the following manner : The em- 

 bryonic ovary is covered with short columnar cells, or the so-called germ- 

 inal epithelium. The cells of this layer undergo proliferation, so as to 

 form several strata, and grow into the ovarian stroma as longer or shorter 



