870 GENERATION. 



vitellus, a single layer of columnar cells ; at the deepest portion of these cells, a homo- 

 geneous basement-membrane is gradually formed ; and the cells undergo a sort of cuticu- 

 lar transformation, becoming finally the vitelline membrane. 



An important point, in this connection, is the question of the existence of pores, or per- 

 forations in the vitelline membrane. As we shall see farther on, there can be no doubt 

 with regard to the actual penetration of the spermatozoids through this membrane, so 

 that they come in contact with the vitellus ; and it is in this way that the ovum is fecun- 

 dated. In the osseous fishes and in mollusks, there seems to be no question with regard 

 to the existence of numerous pores in the vitelline membrane ; but these are not so easily 

 demonstrated in the ova of mammals. Admitting the existence of a micropyle and pores 

 in the vitelline membrane in fishes and mollusks, it is certain that openings are very 

 much more indistinct, if they can be seen at all, in the ova of mammals; still, the fact 

 of the actual penetration of spermatozoids almost of necessity presupposes the presence 

 of orifices. We have often thought, in studying this subject, that it must be difficult, 

 examining a perfectly transparent and homogeneous membrane in water, which would 

 fill up all pores, to distinguish any openings, and we have been disposed to admit their 

 presence, mainly because the spermatozoids are known to pass through. The idea of 

 their existence in mammals certainly receives support from analogy with the lower 

 orders of animals. 



The vitellus, called the principal yolk or the formative yolk, contains the elements 

 which are to undergo development into the embryon. It is composed of a semifluid 

 mass, containing, in addition to the germinal vesicle, numerous granules. Some of these 

 granules are large, strongly-refracting, globular bodies, which are so bright and so numer- 

 ous, that they obscure the other parts of the vitellus. Between these, are numerous albu- 

 minoid granules, which are much smaller and not so distinct. 



The germinal vesicle, sometimes called the vesicle of Purkinje, is the enlarged nucleus 

 of the primordial ovum. It is a clear, globular vesicle, about 7 ^ of an inch in diameter, 

 embedded in the vitellus, its position varying in different ova. It presents in its interior 

 a number of fine granules, and a large, dark spot, called the germinal spot, or the spot 

 of Wagner, which measures about -^Vo of an inch in diameter. This spot corresponds 

 to the nucleolus of the primordial ovum. In mammals, the mature ovum contains but 

 one germinal vesicle and one germinal spot. The various points we have described are 

 illustrated in Fig. 280. 



Discharge of the Ovum. 



A ripe Graafian follicle measures from f to \ of an inch in diameter and presents a 

 rounded elevation, containing a plexus of blood-vessels, upon the surface of the ovary. 

 At its most prominent portion, is an ovoid spot, in which the membranes are entirely free 

 from blood-vessels. At this spot, which is called the macula folliculi, the coverings finally 

 give way, and the contents of the follicle are discharged. For a short time anterior to 

 the rupture of the follicle, important changes have been going on in its structure. In 

 the first place, the non-vascular portion, situated at the very surface of the ovary, under- 

 goes fatty degeneration, by which this part of the wall becomes gradually weakened. At 

 the same time, at the other portions of the follicle, there is a growth of cells, which pro- 

 ject into the interior, and an extension, into the interior, of blood-vessels in the form of 

 loops. These changes, with an increase in the pressure of liquid and the fatty degen- 

 eration of the macula, cause the follicle to burst ; and, with the liquid, the discus prolige- 

 rus and the ovum are expelled. The formation of a cell-growth in the interior of the 

 follicle is really the beginning of the corpus luteum ; and this occurs some time before 

 the discharge of the ovum takes place. It is a disputed question whether or not a 

 haemorrhage occurs into the follicle at the time of its rupture. This may, and undoubtedly 

 does sometimes occur, but it cannot be regarded as constant and has been denied by 

 many observers. 



