THE GERMINAL VESICLE. 721 



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

 which is composed of granules and globules of various 

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

 smaller granules, which are the more numerous, resemble 



in their appearance, as well as 



Fig. 202* 

 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, 

 according to Bischoff, greatest in 

 the ova of carnivorous animals. In 

 the human ovum their quantity is comparatively small. 



In the substance of the yelk is imbedded the 

 vesicle, or vesicula germinativa (figs. 202, 203). 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 ascertained, 

 owing to the difficulty of isolating it ; but it is probably 

 about yig- 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 sub- 

 stance, of a yellowish colour, strongly refracting the rays 

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

 WTTO- to IT ATT of an incn (Wagner.) 



* Fig. 202. 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. 



3 A 



