Fig. 13.—A. Egg of the Dog, "with the vitelline membrane burst, so as to give exit to the yelk, the germinal vesicle (a), and its included spot (b).
B. C. D. E. F. Successive changes of the yelk indicated in the text. After Bischoff.
The egg, or 'Ovum,' is originally formed within a gland, from which, in due season, it becomes detached, and passes into the living chamber fitted for its protection and maintenance during the protracted process of gestation. Here, when subjected to the required conditions, this minute and apparently insignificant particle of living matter, becomes animated by a new and mysterious activity. The germinal vesicle and spot cease to be discernible (their precise fate being one of the yet unsolved problems of embryology), but the yelk becomes circumferentially indented, as if an invisible knife had been drawn round it, and thus appears divided into two hemispheres (Fig. 13, C).
By the repetition of this process in various planes,
