746 



HANDBOOK OF PHYSIOLOGY. 



embedded in the soft stroma of fine connective tissue and unstriped 

 muscle form here the cortical layer; they are sometimes called ovisacs. 



Each of the small follicles of this layer has an external membranous 

 envelope, or membr ana propria. This envelope or tunic is lined with a 



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Fig. 444. Section of the ovary of a cat. A, germinal epithelium; B, immature Graafian 

 follicle; C, stroma of ovary; D, vitelline membrane containing the ovum; E, Graaflan follicle 

 showing lining cells; F, follicle from which the ovum has fallen out. (V. D. Harris.) 



layer of nucleated cells, forming a kind of epithelium or internal tunic, 

 and named the membr ana granulosa. The cavity of the follicle is filled 

 up by a nucleated mass of protoplasm inclosed in a very delicate mem- 

 brane, which is the Ovum. The large spherical nucleus contains one 

 or more nucleoli. The nucleus is known as the germinal vesicle, and 

 the nucleolus as the germinal spot. 



The central portion of the stroma of the ovary extends from the cor- 

 tical layer to the hilum of the organ, at which enter the numerous 

 arteries, fibrous tissue, and unstriped muscle, forming a highly vascular 

 zona vasculosa. Within this central zone are contained the fully-devel- 

 oped Graafian follicles, varying in size however, but considerably larger 

 than those of the cortical layer. In these follicles the cavity is not 

 nearly filled by the ovum, which is attached at one side to the zona 

 granulosa by a collection of small cells, the discus proligerus, the 

 remainder of the cavity being filled with fluid, the liquor folliculi. The 

 envelope of the ovum, or zona pellucida, is much thicker. The zona 

 granulosa is formed of several layers of cells, instead of one only. Its 

 menibrana propria is much thicker, so as to form a distinct fibrous in- 

 vestment; the mem brana fibrosa and the blood-vessels surrounding it are 

 numerous, and may be said to form a membrana vasculosa about it. 



The human ovum measures about y^ of an inch (about .& mm.) in 

 diameter. Its external investment, or the zona pellucida, or vitelline 



