522 EMBRYOLOGY OF THE TURTLE. Parr III. 
said in regard to the superficial or outermost layer of mesoblasts, (fig. 8,) which 
have departed from their cumulated arrangement, and present a uniform stratum 
all over the surface of the embryonic dise and of the germinal layer. 
Everywhere the mesoblasts are now spread uniformly, in unbroken continuity 
and in close contact, yet not pressing against each other so as to assume a polyg- 
onal form. Even at this late period, intimately identified as these bodies are 
with the embryo, their fissuration is in many instances not yet complete, (fig. 
8,) judging from the inequality of their size, when compared with their uniformity 
in that respect at a later age (fig. 34, a). In fact it is evidently impossible to 
distinguish between the fissuration of these bodies as yolk cells, and the same 
operation when they have become the cells of which alone the embryo is com- 
posed, at the age to which we have just traced them; for, in the latter case, they 
have still the same more or less dark, oily outline, with some, here and there, 
containing one or two waxy bodies, (fig. 5a, 8,) entoblasts. By the time, how- 
ever, that the primitive stripe (Pl. 11, fig. 3, 4) has begun to form, this hetero- 
geneous aspect has disappeared; and the mesoblasts, the primitive embryonic cells, 
—as we may now call them, in reference to their being the origimal constituents 
of the embryo,—are of a nearly uniform size (PI. 9a, fig. 34, a) throughout the 
upper surface of the young animal, and the exterior of the germinal layer. 
Here we have, at last, an indisputable series of facts, the succession of which 
is unbroken, showing the origin and nature of what constitutes the primitive cel- 
lular basis of the germ. These facts are enough to establish the identity of the seg- 
mented mesoblasts of yolk cells with those cells which are primarily arranged surface 
to surface to build up the embryo. There is now no room left for the supposition 
that the Purkinjean vesicle takes a part in the operation.’ The idea is negatived 
without directly referring anew to the mode of development and the final disap- 
pearance of that vesicle, when it can be shown, as we have just done, that the 
embryonic dise is entirely composed of yolk-cell mesoblasts after their most minute 
selfdivision. Any further account that may be given of the ulterior changes 
of these cells belongs more properly to that section which treats of the structure 
of the tissues, the histology, 
ov 
of the various organs. 
1 We have already alluded to the exaggerated im- 
portance which has been ascribed to the germinative 
vesicle, and to the erroneous impression conveyed by 
After 
what has been shown in this section respecting the 
its name (p. 481, note 2, and p. 4635, note 1). 
origin of the primitive embryonic cells, we may fairly 
add, that it is now proved that the Purkinjean vesicle 
takes no part in the formation of the embryo, beyond 
ginates, as a dis- 
supplying the region in which it ori 
tinct body, with a larger quantity of albumen than is 
found in other parts of the egg. The whole process 
thus appears like a succession of isolations and recom- 
binations of the oleaginous and albuminous substance 
of which the yolk is composed, with a prevalence of 
the albumen at one pole of the egg, where the embry- 
onic dise arises, and a more extensive accumulation of 
the oleaginous mass at the other pole, where the so- 
called vegetative systems of organs originate. 
