[80] 



OVUM. 



addition to the ovum of the rabbit, the diame- 

 ter of which is thus increased between two 

 and three times, so as to give it somewhat the 

 aspect of the ovum of a Batrachian in minia- 

 ture ; and in the dog it has appeared to me 

 that the increased thickness and more opaque 

 and flocculent roughness of the surface of the 

 zona were sufficient proofs of a new deposit 

 having taken place. This deposit, no doubt, 

 becomes very completely incorporated with 

 the substance of the zona, and is not easily to 

 be distinguished from it ; but in one or two 

 instances I have thought that I was able to 

 perceive a line of demarcation between them. 

 Several of Bischoff's very faithful figures seem 

 to me even to represent this deposit as it has 

 occurred on the ova of the dog. But his 

 statements in his work on the Development of 

 the Guinea- Pig * are so precise against the 

 occurrence of such a deposit, that further ob- 

 servations will be required fully to determine 

 the question whether it is of constant oc- 

 currence or essential to the formation of the 

 chorion. 



Later observations lead me to think that I 

 may have been in error in supposing that the 

 villi of the chorion grow directly from the al- 

 buminous deposit. These villi, which, as I have 

 said, form a most characteristic feature of the 

 external covering of the mammiferous ovum in 

 the course of development, begin to be formed 

 only when the ovum has reached the cavity of 

 the uterus. The time of the formation of these 

 villi, as well as their size, varies, however, con- 

 siderably in different animals, and probably 

 also to some extent in the same animal. -j- 

 They are developed from the external surface, 

 and their structure is at first nearly homo- 

 geneous, or at least only slightly granular ; 

 they afterwards acquire a cellular structure, 

 and in the course of foetal development be- 

 come at an early period the seat of a compli- 

 cated vascular growth, by which the relations 

 of the maternal parent and offspring are main- 

 tained through utero-gestation. But the fuller 

 description of this part of the growth of the 

 chorion belongs rather to the history of de- 

 velopment after fecundation. My present 

 object has been only to show the relation of 

 this membrane to the zona or ovarian cover- 

 ings of the ovum. 



The contents of the ovum or parts within 

 the zona consist of the yolk-mass or yolk- 

 substance, and the germinal vesicle. The first 

 of these constitutes a spherical mass of varia- 

 ble consistence, in which granules, or molecules 

 and globules of various sizes, from the most 

 minute up to about ^oo or 4oW> are sus- 



Fig. 59*. 



pended in a fluid. The proportion of the gra- 

 nules to the fluid varies to a considerable ex- 

 tent in different animals, the ovum being much 

 more opaque, and usually of a dull-yellow co- 

 lour when the granules are in large quantity, 

 as is the case in most Carnivora, and may be 

 easily seen in the dog or cat. The clear fluid in 



* Entwickelungsg. des Meerschweinchens, 4to. 

 Giesseu, 1852. 



f Barry and Bischoff. 



Ovarian ovum of the Rabbit. (From Coste.) 



a, ovarian ovum extracted from a nearly ripe 

 Graafian follicle, and freed from the adherent 

 granular cells; the close set granules of the yolk 

 substance, among which the germinal vesicle is 

 perceived with a slightly oval macula or nucleus, 

 are well represented. 



b, the same burst by pressure ; the yolk granules 

 adhering together by a viscid clearer fluid sub- 

 stance are seen escaping from the large aperture in 

 the zoua along with the germinal vesicle. 



which the granules are suspended varies also in 

 its consistence, being of a marked viscid qua- 

 lity in some instances, and thin and limpid in 

 others; so that in some animals, when the zona 

 is punctured, the yolk-substance flows freely 

 out, while in others, and this is the case in 

 the human ovum, the yolk-substance holds 

 together as one consistent mass. The yolk- 

 substance does not adhere in the slightest to 

 the interior of the the zona, but on the con- 

 trary is readily detached from it ; and in some 

 instances, in the entire unimpregnated ovum, a 

 space is seen between the yolk-substance and 

 the zona, formed apparently by the imbibition 

 of water between them. This separation be- 

 tween the yolk-substance and zona appears 

 to be, at a later period, a constant and pro- 

 bably important change in connection with 

 fecundation and development. 



The granules of the yolk-substance are ge- 

 nerally rather more densely set together and 



