346 Agassiz on the Embryology of the Turtle. 
and many to disappear altogether, whilst new ones, with rounded 
angles and more equilateral, develop in place of these. 
The Purkinjean vesicle—We have already described the mode 
of origin of the Purkinjean vesicle, and mentioned the appear- 
ance of the Wagnerian vesicles. These last are developed at 
first few in number, but subsequently they cover, like drops of 
dew, the whole inner surface of the wall of the Purkinjean 
vesicle. Like all cell development heretofore described, the 
Wagnerian vesicle begins with a faintly visible conglomeration 
of particles, which eventually obtains a well defined outline. 
e Valentinian vesicle is remarkable for having very little re- 
fractive power, so as to appear like-a flat disc in the midst of 
the parent cell, the Wagnerian vesicle. ae 
y the time the egg has become one quarter of an inch in 
diameter, the Wagnerian vesicles disappear and give place to an 
almost total homogeneity of contents in the Purkinjean vesicle. 
From this time forward the latter grows more and more albu- 
minous, and may be hardened, by boiling the egg, so as to be 
taken up on the point of a knife. This albuminousness 1s s0 
The Zona pellucida——The zona pellucida, which eventually 
takes the place of the yolk sac, when the latter becomes resorbed, 
* “The clear space, observable in the egg of various animals just previous 7 
segmentation, to which the name of “embryo iven, from 1 vega’ 
intimate connection with the formation of the germ, may be identical with ro “es 
ve 
moreover, that the Purkinjean vesicle is not to be so definitely separated, as regarc 
its tial element, from the immediately juxtaposed substance of similar e802 
ance, but should rather be looked upon as the crowning point of albuminous eed 
centration, to which the opposite side of the egg stands in the reverse extreme : 
highly oleaginous nature. A reference to the mode of origi 
is conclusively ; for it is developed as a ary n in the egg- 
evolution, and not as t ary basis to a succeeding structure ever after nce 
pats: import, and leading, as some would jt, to pn 
te of superior i 
coming in the end the essential element in the genesis of the embryo. pani 
of origin alone, we maintain, is sufficient to show that the Me foundation 2 
refore, loses all its advocated claims to preponderance ove hye 
constituents; to say nothing of the fact that it takes no part in the building | th 
the blastoderm, excepting that its discharged contents may become abso i 
i ie inte es of su y 
tic and exosmotic 
and the albuminous matter in which t nough, the region - 
vesicle exhibits a specialized nature; it is there that the embryo first develo 
tain of its characteristics, previous to i ension; but it does mr 
that Purkinjean vesicle is situated thereabout, it is the basis 0 
lution, or in any way causatively ith i contrary, its pr 
is itself result of certain , for instance, the concentra of 
albumen in that direction; and its disappearance is —* 
consummation of these tendencies,” 
