OVUM. 



[81] 



Ovum of Mammalia and of the Human 

 Species. There is a remarkable uniformity 

 in the size, structure, and relations of the 

 ovarian ovum in the whole class of Mammalia, 

 with the exception of the families of Marsu- 

 piata and Monotremata ; in the last of which 

 especially there is an approach to the oviparous 

 type. We shall first consider the more com- 

 mon form of the rnammiferous ovum. Of this 

 the most marked characteristics are, as has 

 already been stated, the very small size in 

 proportion to the ovarian follicle, the finely 

 granular yolk-substance, and the dense, clear, 

 and firm external covering or zona pellucida. 



The Graafian follicles, or ovarian vesicles, 

 in which the ova are situated, attain, when 

 mature, a size of from -^ to or even an 

 inch, varying in size in some measure with, 

 but not in exact proportion to, the stature of 



the animals. In the human ovary these fol- 

 licles are firm spheroidal sacs, which attain 

 when mature an average size of about of an 

 inch. In the ovaries of women, during the 

 child-bearing period, a number of smaller folli- 

 cles lie throughout the greater part of the 

 substance of the ovary ; the more developed 

 follicles being usually placed towards the free 

 surface, but at some little distance from it. As 

 they enlarge and approach maturity, the ova- 

 rian substance appears to give way to them, 

 or to become gradually thinner between the 

 follicles and the outer surface of the ovary, 

 so as at last to leave almost nothing but the 

 covering membranes of the ovary at the most 

 projecting part. Even when of their full size, 

 however, the Graafian follicles of the human 

 subject and of most animals do not project 

 much beyond the general surface of the ovary; 



Fig. 54*. 



cv 



Mamnriferous Ovum. 



A. (From Coste.) Human ovary enlarged four diameters, partially dissected at ooo, to show the 

 Graafian follicles in the ovarian stroma : one of these, more advanced, has had its double tunic o v 

 cut into and reflected ; the granular membrane m g has also been partially opened, showing the 

 thickened portion or granular disc da, in which the ovum is imbedded near the most projecting 

 part. At o v', another Graafian follicle has been burst, and the ovum in its granular disc is seen 

 expelled from it. 



B. Transverse section of the human ovary, to show the general arrangement of the developed 

 Graafian follicles towards the surface ; twice the natural size. 



c. Diagrammatic representation on an enlarged scale, in section, of two Graafian follicles, in dif- 

 ferent stages of advancement in the ovary of a mammifer. p, peritoneal covering of the ovary ; 

 * t, ovarian stroma ; o t>, the two layers of the ovisac ; m g, membrana granulosa, near which is 

 the discus granulosus, with the ovum imbedded. 



