These researches have all tended to establish one great law, connected with the early development of the human embryo, and that of other mammiferous animals, viz, that it at first possesses a structure and arrangement analogous to that of animals in a much lower scale of formation: this observation also applies of course to the ovum itself, since a variety of changes take place in it after impregnation, before a trace of the embryo can be detected. At the earliest periods, the human ovum bears a perfect analogy to the eggs of fishes, amphibia, and birds; and it is only by carefully examining the changes produced by impregnation in the ova of these lower classes of animals, that we have been enabled to discover them in the mammalia and human subject. As the bird's egg, from its size, best affords us the means of investigating these changes, and as in all essential respects they are the same in the human ovum, it will be necessary for us to lay before our readers a short account of its structure and contents, and also of the changes which they undergo, after impregnation. In doing this we shall merely confine ourselves to the description of what is applicable to the human ovum. [Illustration: _Section of a hen's egg within the ovary._ _a_ The granulary membrane forming the periphery of the yelk. _b_ Vesicle of Purkinje imbedded in the cumulus. _c_ Vitellary membrane. _d_ Inner and outer layers of the capsule of the ovum. _e_ Indusium of the ovary.] The egg is known to consist of two distinct parts, the vitellus or yelk surrounded by its albumen or white; to the former of these we now more particularly refer. The yelk is a granular albuminous fluid, contained in a granular membranous sac (the _blastodermic membrane_) which is covered by an investing membrane called the _vitelline membrane_ or _yelk-bag_. The impregnated vitellus is retained in its capsule in the ovary, precisely as the ovum of the mammifera is in the Graafian vesicle. The whole ovary in this case has a clustered appearance, like a bunch of grapes, each capsule being suspended by a short pedicle of indusium. [Illustration: _a_ Vitelline membrane _b_ Blastoderma. _From T. W. Jones._] In those ova which are considerably developed before impregnation, the granular blastermodic membrane is observed to be thicker, and the granules more aggregated at that part which corresponds to the pedicle, forming a slight elevation with a depression in its centre, like the cumulus in the proligerous disc of a Graafian vesicle. This little disc is the blastoderma, germinial membrane or cicatricula; in the central depression just mentioned is an exceedingly minute vesicle first noticed by Professor Purkinje of Breslau, and named after him: in more correct language it is the _germinal vesicle_.