OF THE OVUM AND ITS DEVELOPMENT. 891 



the Graafian follicle the ovum becomes surrounded by a series of secondary 

 formations, which are collectively to be regarded as products of the follic- 

 ular epithelium. To these belong the vitelline membrane or membrane 

 inclosing the yolk, which on account of its thickness and peculiar trans- 

 parency has received in Mammalia the distinctive appellation of Zona pel- 

 lucida (b, Fig. 310; mv, Fig. 311), and the food yolk or secondary yolk. 

 The zona pellucida in Mammals appears to be perforated by numerous fine 

 pores, which confer upon it a striated aspect; it is highly elastic, and when 

 torn with needles the edges roll back and the contents issue in a stream. It 

 resists to a considerable degree the action of acids and alkalies. When ma- 

 ture the Human Ovum is extremely minute; not measuring above y^th of 

 an inch in diameter, and being sometimes no more than half that size. The 

 diameter of the germinal vesicle of the Human Ovum has not yet been as- 

 certained, owing to the difficulty of isolating it from the yolk; in the ovum 

 of the Rabbit, it is about ? Aoth and in the Guinea-pig about y^^th of an 

 inch ; and that of the germinal spot, in the Mammalia generally, is from 

 3- g Voth to 7, T 'oT) tn of an inch. 



738. In Plate I, Fig. 4, is represented the condition of the Graafian vesi- 

 cles in various stages of development, as they are seen imbedded in the 

 fibrous stroma of the ovarium, in a thin slice from the ovary of a sow three 



FIG. 311. 



Constituent parts of Mammalian Ovum: A, entire; B, ruptured, with the contents escaping; mv, 

 vitt-lline membrane ; j, yolk ; v g, germinal vesicle ; I g, germinal spot. 



weeks old. The ovum at first occupies the centre of the Graafian vesicle, 

 but it subsequently removes to its periphery; and, when the contents of the 

 ovisac are undergoing maturation, prior to their escape, the ovum is always 

 found on the side of it nearest to the surface of the ovary. The proper Ovisac, 

 whose wall is formed of a non-vascular membrane, is surrounded by a vas- 

 cular layer, which is formed by a condensation of the ordinary stroma of 

 the ovarium; it is this which is usually described as the outer layer of the 

 Graafian vesicle. 



739. A continual change seems to be taking place in the contents of the 

 Ovarium during the greater part of life; certain of the Ovisacs or Graafian 

 vesicles, and their contents, successively arriving at maturity, whilst others 

 degenerate and die. According to the valuable inquiries of Dr. Ritchie, 1 it 

 appears that even during the period of childhood, there is a continual rup- 

 ture of ovisacs and discharge of ova, at the surface of the ovarium. The 

 Ovaria are studded with numerous minute copper-colored macula, and their 

 surface presents delicate vesicular elevations, which are occasioned by the most 

 matured ovisacs; the dehiscence of these takes place by minute puuctiform 



1 London Medical Gazette, 1844. See also Foulis, Brit. Med. Journ., 1875, p. 

 841. 



