72O GENERATION AND DEVELOPMENT. 



and usually in close contact with it, lies the yolk or vitellus 

 which is composed of granules and globules of various 

 sizes, imbeded in a more or less fluid substance. The 

 smaller granules, which are the more numerous, resemble 

 in their appearance, as well as 

 their constant motion, pigment- 

 granules. The larger granules or 

 globules which have the aspect of 

 fat-globules, are in greatest num- 

 ber at the periphery of the yelk. 

 The number of the granules is, 

 4 *' according to Bischoff, greatest in 

 the ova of carnivorous animals. In 

 the human ovum their quantity is comparatively small. 



In the substance of the yolk is embedded the germinal 

 vesicle, or vesicula germinativa (figs. 209, 2IO). This 

 vesicle is of greatest relative size in the smallest ova, 

 and is in them surrounded closely by the yelk, 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 yelk, and comes to be placed near to its 

 surface. Its size in the human ovum has not yet been ascer- 

 tained, owing to the difficulty of isolating it; but it is 

 probably about y^- of an inch in diameter. 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 yelk is situated 

 the germinal spot (macula germinativa) } a finely granulated 

 substance, of a yellowish colour, strongly refracting the 

 rays of light, and measuring, in the Mammalia generally, 

 from ^-gVo to ^Vu of an incn (Wagner). 



* Fig. 209. Ovum of the sow, after Barry, i. Germinal spot. 2. 

 Germinal vesicle. 3. Yelk. 4. Zona pellucida. 5. Discus proligerus, 

 6. Adherent granules or cells. 





