586 DEVELOPMENT OF THE IMPREGNATED EGG. 



CHAPTER VII. 



ON THE DEVELOPMENT OF THE IMPREGNATED 

 EGG SEGMENTATION OF THE VITELLUS BLAS- 

 TODERM1C MEMBRANE FORMATION OF ORGANS 

 IN THE FROG. 



WE have seen, in the foregoing chapters, how the egg, produced 

 in the ovarian follicle, becomes gradually developed and ripened, 

 until it is ready to be discharged. The egg, accordingly, passes 

 through several successive stages of formation, even while still con- 

 tained within the ovary ; and its vitellus becomes gradually com- 

 pleted, by the formation of albuminous material and the deposit of 

 molecular granulations. The last change which the egg undergoes, 

 in this situation, and that which marks its complete maturity, is the 

 disappearance of the germinative vesicle. This vesicle, which is, in 

 general, a prominent feature of the ovarian egg, disappears but a 

 short time previous to its discharge, or eyen just at the period of 

 its leaving the Graafian follicle. 



The egg, therefore, consisting simply of the mature vitellus and 

 the vitelline membrane, comes in contact, after leaving the ovary, 

 and while passing through the Fallopian tube, with the spermatic 

 fluid, and is thereby fecundated. By the influence of fecundation, 

 a new stimulus is imparted to its growth ; and while the vitality 

 of the unimpregnated germ, arrived at this point, would have 

 reached its termination, the fecundated egg, on the contrary, 

 starts upon a new and more extensive course of development, by 

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



The egg, in the first place, as it passes down the Fallopian tube, 

 becomes covered with an albuminous secretion. In the birds, as we 

 have seen, this secretion is very abundant, and is deposited in suc- 

 cessive layers around the vitellus. In the reptiles, it is also poured 

 out in considerable quantity, and serves for the nourishment of the 

 egg during its early growth. In quadrupeds, the albuminous matter 

 is supplied in the same way, though in smaller quantity, by the 



