448 OF GENERATION AND DEVELOPMENT. 



membrane, or yolk-bag. The vitellus, or yolk, consists chiefly of albu- 

 men and oil-globules ; and floating in this fluid is seen a cell of peculiar 

 aspect, termed the germinal vesicle, upon the wall of which is a very 

 distinct nucleus, termed the germinal spot. The layer of albumen sur- 

 rounding the yolk, and termed the white of the Bird's egg, together 

 with the membrane which envelopes this and forms the basis of the 

 shell, are not added until after the ovum has left the ovarium. They 

 are not present in the eggs of many of the lower Invertebrata ; these 

 consisting merely of the parts which are formed within the ovarium. 



795. The structure of the ovule in Mammals differs in no essential 

 particular from that just described; but the yolk is much less in 

 amount, than in the ovules of Invertebrated animals ; since only the 

 very earliest stages of the development of the embryo are to take place 

 at its expense. The vitelline membrane is of peculiar thickness and 

 transparency ; and as, when the ovum is compressed under the micro- 

 scope, it is seen as a broad transparent belt, it is commonly known as 

 the zona pellucida. We shall find that the ovule, after leaving the 

 ovarium and receiving the fertilizing influence, becomes enclosed, whilst 

 passing through the Fallopian tube, with a layer of albuminous matter, 

 which represents the white of the Bird's egg ; and with an additional 

 fibrous envelope, which corresponds with the membrane enveloping the 

 latter. This fibrous membrane, termed the Chorion, afterwards becomes 

 subservient, however, to various important changes ; by means of which 

 the ovum is again brought into connexion with the parent, to derive 

 from the blood of the latter the materials requisite for the continued 

 development of the embryo. These changes will be described hereafter 

 ( 811, 818). 



796. The Ovisac of Mammalia forms the inner layer of what is 

 termed the Gf-raafian follicle, after the name of its discoverer ; and in- 

 stead of closely enveloping the ovulum, as it does in oviparous animals, 

 it contains in addition to it, a quantity of granular matter, consisting 

 of cells arranged in membranous layers, together with fluid. In the 

 immature ovisac, these cells occupy nearly the whole space between the 

 ovisac and the ovum, but they gradually dissolve away, especially on 

 the side nearest the surface of the ovary ; and at the same time an al- 

 buminous fluid is effused from the deeper part of the ovisac, which 

 pushes before it the residual layer of cells that immediately sur- 

 rounds the ovum (forming the discus proligerus), and thus carries it 

 against the opposite wall. The outer layer of the Graafian follicle is 

 formed by a thickening and condensation of the surrounding paren- 

 chyma of the ovarium ; and it is quite distinct from the ovisac which it 

 envelopes. It is extremely vascular, and is evidently destined to 

 afford to the contained structures the materials for their development, 

 which they receive and appropriate by their own powers of absorption 

 and assimilation. 



797. The Mammalian Ovarium may be seen, even in the foetal ani- 

 mal, to contain immature ova, enclosed within their ovisacs ; and the 

 several parts of the former may be clearly distinguished in those which 

 are in the more advanced stages of development. It appears that, 

 during the period of childhood, there is a continual rupture of the ovi- 



