ACTION OF THE FEMALE. 701 



That the presence of Corpora Cephaloidea of the second order is to be re- 

 garded as indicative of conception. 3. That the presence of the Corpora 

 Cephaloidea of the first order, or of Corpora Mbida, cannot be regarded as 

 in the least degree indicative of Conception ; as they may result from the 

 simple discharge of an Ovum, in the ordinary course of those changes to 

 which the Ovarium is subject. The excess of Corpora Albida above every 

 other appearance is due, not merely to their being an ordinary result of the 

 discharge of unimpregnated Ova ; but also to the frequency of their produc- 

 tion as degenerated forms (so to speak) of the Corpora Cephaloidea and Cor- 

 pora Rubra of the gravid female ; and also to their occasional existence as, 

 from the first, the only Ovarian change following upon Conception. 



915. The object of the changes which have been already described, is to 

 bring the Ovum within reach of the fecundating influence ; and to convey it 

 into the Uterus after it has been fertilized. We have now to consider the 

 changes in the Ovnm itself, Avhich take place during the same epoch. At 

 about the same period that the Ovum moves towards the periphery of the 

 Graafian follicle, the Germinal Vesicle moves towards the periphery of the 

 yolk-bag ; and it always takes up its position at the precise point of the Zona 

 Pellucida which is nearest the Ovisac, and which is closest, therefore, to the 

 surface of the Ovary. Moreover, the Germinal Spot is always on that part 

 of the Germinal Vesicle, which is in closest contact with the Zona Pellucida. 

 (See a, Figs. 9 and 10, Plate I.) Thus, the Germinal Spot is very near the 

 exterior of the Ovary ; but is separated from it by the peritoneal coat of the 

 latter, by a thin layer of its stroma forming the external layer of the Graafian 

 follicle, by the ovisac forming its internal membrane, and by the zona pellu- 

 cida. We have already seen how the obstacle interposed by the three former 

 to the entrance of the Spermatozoon, is overcome ; we shall presently find 

 that the Zona Pellucida undergoes a similar change. 



916. Whilst the Ovum is being prepared for fecundation, a series of very 

 important actions takes place in the Germinal Vesicle. The exterior or peri- 

 pheral portion of the Spot, which previously consisted of a collection of very 

 minute granules, begins to develope itself into a ring of new cells of extreme 

 delicacy (Fig. 9, ) ; these gradually enlarge, and a second ring of cells is 

 developed within it, pushing the first-formed cells further away from the cen- 

 tre. Many successive rings of cells are thus formed ; and at last the whole 

 Germinal Vesicle is filled with them, as shown at b, Fig. 10. Still there 

 remains a pellucid space in the centre of the Germinal Spot (resembling that 

 seen at a, Fig. 12) ; in which no cells are developed. The first-formed cells 

 that have been pushed outwards, are so much compressed by those subse- 

 quently formed, as frequently to undergo liquefaction ; and during the time 

 that the Ova are being matured for fertilization, there is a continual new pro- 

 duction of cells at the centre, and a degeneration at the circumference. At 

 the same time, the Yolk undergoes changes somewhat analogous ; for it 

 ceases to contain separate oil-globules ; and large elliptical discs or cells are 

 seen in it, especially just beneath the Zona Pellucida (Fig. 9, c).* Here, too, 

 the formation of new cells takes place from the periphery towards the centre ; 

 the peripheral ones gradually undergo liquefaction, as is seen in the outer 

 layer of those in Fig. 10, which are becoming indistinct ; and they are replaced 

 by a new layer pushed outwards from the centre. The same process sub- 

 sequently continues in the Yolk, for some time after fecundation ; and this 

 not only in regard to the yolk as a whole, but in respect to its individual cells, 



* It is to be remembered that the observations of Dr. Barry, here quoted, were made on 

 the Rabbit : and are, therefore, probably applicable equally to other Mammalia, but not to 

 Oviparous Animals. 



59* 



