DEVELOPMENT OF OVUM. 



565 



carnivorous animals. In the human ovum their quantity is 

 comparatively small. 



In the substance of the yelk is imbedded the germinal vesicle, 

 or vesicula germinativa (Figs. 209, 210). This vesicle is of 

 greatest relative size in the smallest ova, and is in them sur- 

 rounded 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 isolat- 

 ing it; but it is probably about ^^ of an inch in diameter. 

 It consists of a fine, transparent, structureless membrane, con- 



Diagram of a Graafian vesicle, containing an ovum. 1. Stroma or tissue of the 

 ovary. 2 and 3. External and internal tunics of the Graafian vesicle. 4. Cavity of 

 the vesicle. 5. Thick tunic of the ovum or yelk-sac. 6. The yelk. 7. The germinal 

 vesicle. 8. The germinal spot. 



taining 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 color, strongly refracting the rays of 

 light, and measuring, in the Mammalia generally, from ^g 1 ^ 

 to ^7770 f an i ucn (Wagner). 



Such are the parts of which the Graafian follicle and its 

 contents, including the ovum, are composed. The diagram 

 (Fig. 210) represents them in their relative positions when 

 mature. With regard to the mode and order of development 

 of these parts there is considerable uncertainty; but it seems 

 most likely that the ovum is formed before the Graafian vesi- 

 cle or ovisac. 



48 



