CHAPTEE VII. 



DEVELOPMENT OF THE IMPREGNATED EGG SEG- 

 MENTATION OF THE VITELLUS BLASTODERM- 

 FORMATION OF ORGANS IN THE FROG. 



THE egg, while still contained within the ovarian follicle, passes 

 through a series of consecutive changes, by which it is finally brought 

 to the condition of maturity. During this period it increases in size, 

 from the insignificant dimensions which it presents in the earlier stages 

 of its formation, to those of its complete development as an ovarian 

 egg. The vitellus, at first transparent and colorless, is not only en- 

 larged, but becomes more or less granular and opaque by the deposit 

 of new material in a different form ; and in birds and reptiles it assumes 

 a distinctive hue, which is generally orange or yellow. These modifica- 

 tions are due to the spontaneous growth of the egg and the parts in 

 which it is inclosed ; and they mark a continuous process of develop- 

 ment taking place independently in the generative organs of the female. 

 The last change which occurs in the ovarian egg, and that which indi- 

 cates its complete maturity, is the disappearance of the germinative 

 vesicle. This body, which is in general a distinctive feature of the ova- 

 rian egg, disappears a short time previous to its expulsion, or even when 

 it is just on the point of leaving the Graafian follicle. 



The egg, therefore, at the time of its discharge from the ovary, con- 

 sists solely of the mature vitellus, inclosed in the vitelline membrane ; 

 and in this condition it meets with the spermatozoa, usually in some 

 part of the Fallopian tube. By the contact of the male elements, and 

 their union with its own substance, a new stimulus is imparted to its 

 growth; and while, if unimpregnated, its vitality, on arriving at this 

 point, would have reached its termination, the fecundated egg, on the 

 contrary, starts upon a more extensive course of development, by which 

 it is finally converted into the body of the young animal. 



Deposit of Albuminous Layers in the Fallopian Tube. The egg, in 

 the first place, as it passes down the Fallopian tube, becomes covered 

 with an albuminous secretion. In birds, this secretion is very abun- 

 dant, and is deposited in successive layers around the vitellus, forming 

 the so-called " white of egg " In reptiles, it is also poured out in con- 

 siderable quantity, and serves for the nourishment of the egg during its 

 early growth. In mammalians, albuminous matter is supplied in the 

 same way, though in smaller quantity, by the mucous membrane of the 

 Fallopian tube, and envelops the egg in a layer of nutritious material. 

 This albuminous layer, although its absolute quantity is very small, is 



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