GENERATION AND DEVELOPMENT. 237 



In order to examine an ovum, one of the Graafian vesicles, it matters 

 not whether it be of small size or arrived at maturity, should be pricked, 

 and the contained fluid received upon a slide. The ovum then, being 

 found in the midst of the fluid by means of a simple lens, may be further 

 examined with higher microscopic powers. Owing to its globular form, 

 however, its structure cannot be seen until it is subjected to gentle pressure. 



The human ovum measures about T J-g- of an inch. Its external invest- 

 ment is a transparent membrane, about ^Vo of an inch in thickness, which 

 under the microscope appears as a bright ring (4, Fig. 

 395), bounded externally and internally by a dark out- 

 line; it is called the zona pellucida, or vitelline mem- 

 brane. It adheres externally to the heap of cells con- 

 stituting the discus proliger us. Within this transpar- 

 ent investment or zona pellucida, and usually in close 

 contact with it, lies the yolk or vitellus which is 

 composed of granules and globules of various sizes, FIG. 395.-Ovumofthe 

 imbedded in a more or less fluid substance. The 

 smaller granules, which are the more numerous, re- 

 semble in their appearance, as well as their constant ceUs. re (Bar?yo nules r 

 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 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 yolk is imbedded the germinal vesicle, or vesi- 

 cula germinativa (2, Fig. 395). This 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 is about ^-Q 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 yolk is situated the germinal spot (macula 

 germinativa}, a finely granulated substance, of a yellowish color, strongly 

 refracting the rays of light, and measuring about ^fo$ of an inch in 

 diameter. 



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

 cluding the ovum, are composed. 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 vesicle or 

 ovisac. 



With regard to the parts of the ovum first formed, it appears certain 

 that the formation of the germinal vesicle precedes that of the yolk and 



