. 
Cuap. II. THE WHOLE EGG IS THE EMBRYO. 529 
afterward becomes an essential part of the “embryo,” as the latter extends itself 
in the form of a germinal layer and a vascular area, not only all over the 
surface of the yolk, but, in the case of the area vasculosa, through the whole 
vitelline mass, the latter becoming a great spongy network of bloodvessels, formed 
by the lateral apposition of the cells composing this large body. This vascular 
mass is finally drawn into the body, and, though gradually disappearing by resorp- 
tion, remains for nearly six months after birth, as one of the essential portions 
of the organization of the freely moving animal. 
Because a part is more or less separated from the main body, it does not 
follow that it should be considered as an appendage or an unessential portion of 
its structure. The bladder, at one time, hangs out in a saccular form as an 
allantois, extending far beyond its subsequent relations, and yet it is an organ of 
the embryo. No one would deny that the legs are an integral part of an ani- 
mal because they extend beyond the bulk of the body; no one would hold that 
the young teeth, which, after a certain age, are discharged from their capsule, were 
not essentially a part of the body because they eventually disappear; no one 
would assert that the menses are not a characteristic physiological phenomenon of 
the animal system because they cease at a certain age; or that the ovaries, 
because they are resorbed at this period, were mere transient accessories of the 
organization. As if life were ever at a stand-still, a stereotyped machine, hewed 
and hammered out and put up to perform a certain uniform work, never changing 
from the time it is built till it falls to pieces by wear and decay! No; not so. 
We may truly say that life is embryonic all through; embryonic, in the sense 
that changes go on in the adult as in the young, and oftentimes quite as exten- 
sively as in the unborn or just born animal. From the moment that the egg is 
isolated, a new individual life commences; the animal potentially exists. Nor are 
we by any means to suppose, that the yolk, because it floats freely for a while, 
is a mere vitelline substance, and not an integral part of the embryo. Does 
not the blood float freely in the adult body? and does it not originate in the 
embryo as a loosened mass of yolk-cell mesoblasts, (Pl. 19, fig. 6,) separated from 
the sides of the channels, which, after having been hollowed out in the thickness 
of the intestinal layer, form bloodvessels? And yet, who will deny that this fluid 
nutritive substance, in contradistinction to the ‘ em- both cases the nourishment is taken up by vessels 
bryo,” which is placed above it, the latter increasing through the process of endosmosis. The yolk is 
in size as the former supplies the materials. (Comp. never appropriated by a process of digestion. There 
Part. I., Sect. 1, p. 181, and Sect. 6, p. 229.) This, we ~ was a time in the history of Embryology when the 
will admit, is true; but only in the same sense that the terms “ego” and “embryo” were synonymous; we 
stomach, as an independent organ of the body, bears have to go back to it, now that we know how gradu- 
the means of existence to the whole organism. In ally the egg is transformed into a distinct embryo. 
67 
