FERTILIZATION OF OVUM. 101 



substance of the yolk is a small vesicular body, the germinal vesicle (vesicle of Pur- 

 kinje). the nucleus of the cell ; and this contains as its nucleolus a small spot, the 

 / i/t'/'ininatii'a. or germinal spot of "NN agner. 



The zona pellucida, or vitelline membrane, is a thick, colorless, transparent mem- 

 brane, which appears under the microscope as a radially striated membrane, 

 bounded externally and internally by a dark outline. The stride are believed to 

 be minute pores, and are regarded as the channels by which nutritive particles 

 are admitted into the interior of the ovum, and possibly the way by which the 

 spermatozoa gain access into the interior of the ovum, after the rupture of the 

 Graafian follicle. The presence of these stride has given to the zona pellucida 

 the name of zona radiata, or striated membrane of the ovum. 



The yolk consists of granules or globules of various sizes imbedded in a finely 

 reticulated matrix of protoplasm. The smaller granules resemble pigment; the 

 larger granules, which are in the greatest number at the periphery, resemble fat- 

 globules. In the human ovum the number of granules is comparatively small. 

 Before and immediately after fertilization the cell protoplasm shows distinct 

 movements of contraction and expansion. 



The germinal vesicle consists of a fine, transparent, structureless membrane 

 containing a clear matrix, in which are occasionally found a few granules. It is 

 about T ^~Q of an inch in diameter, and in immature ova lies nearly in the centre 

 of the yolk ; but as the ovum becomes developed it approaches the surface and 

 enlarges somewhat. 



The germinal spot occupies that part of the periphery of the germinal vesicle 

 which is nearest to the periphery of the ovum. It is opaque, of a yellow color, 

 and finely granular in structure, measuring from ^j 7 to j^T ^ an i ncn - 



The phenomena attending the discharge of the ova from the Graafian vesicles, 

 since they belong as much or more to the ordinary function of the ovary than to 

 the general subject of the growth of the body, are described with the anatomy of 

 the ovaries on a subsequent page. 



Either before its escape from the Graafian follicle or immediately after, the 

 ovum undergoes a peculiar change, which results in the formation of one or more 

 peculiar bodies, the polar globules of Robin, and also of another body, which 

 is named the "female pronucleus." The manner in which these bodies are 

 developed from the germinal vesicle is briefly as follows : Usually before the 

 rupture of the Graafian follicle, but after the ovum has become mature or ripe, 

 a portion of the germinal vesicle with a small amount of surrounding protoplasm 

 is protruded outside the yolk, but still remains within the vitelline membrane ; 

 this forms a small globular mass and constitutes the first polar globule. After 

 a time, generally not till the ovum has entered the tube, a second protrusion of a 

 portion of the germinal vesicle takes place, and forms a second polar globule. 

 We have thus about three-quarters of the germinal vesicle extruded from the 

 yolk and about one-quarter remaining behind, and at the ejection of each of these 

 bodies a visible shrinking of the yolk takes place. The portion of the germinal 

 vesicle which remains behind recedes from the surface toward the centre of the 

 yolk and assumes a spherical form, and is now termed the i% female pronucleus. 

 All these changes, it must be understood, occur at each expulsion of an ovum, and 

 are quite independent of fecundation. 



The first changes in the ovum which take place at the time of conception 

 appear to be as follows : A spermatozoon penetrates the ovum, and comes into 

 contact with the portion of the germinal vesicle remaining in the ovum. It 

 seems as if this normally occurs in the Fallopian tube. 1 but it is possible that it 

 sometimes occurs before the ovum has entered the tube, or after it has passed 

 through the tube and reached the cavity of the uterus : abnormally it may even 



1 Many physiologists, as Bischoff and Dr. M. Barry, taught that the ovum is fecundated in the 

 i vary, but the reasoning of Dr. Allen Thomson appears very cogent in proving that the usual spot 

 at which the spermatozoa meet the ovum is in the tube, down which it slowly /ravels to the uterus, in 

 its course becoming surrounded hy an albuminous envelope derived from the walls of the tube. 



