Agassiz on the Embryology of the Turtle. 343 
The physiognomy of the egg in its younger stages is so pecu- 
liar, with its thick dark outline, brilliant, strongly refractive, 
itcannot be mistaken for one of the cells, of the corpus graafia- 
hum, which press upon it from all sides. ‘The initial form of 
| an egg is a dark, oily looking, granule-like, spherical body, situ- 
among the interstices* of the cells of the corpus graafianum, 
As the latter not only, but even their nuclei surpass such an 
egg in size by several diameters, it is superfluous to debate the 
—* whether the egg may not be the nucleus of a cell of 
€ generating organ.” 
_ At first the egg has no wall about it, but finally by a differen- 
tiation of the superficial particles a consistent envelop is elabo- 
tated and answers to the name of the vitelline sac. 
The origin of the Purkinjean vesicle —About this time, or soon 
after, the Purkinjean vesicle—germinal vesicle oftentimes called 
—becomes visible, by a condensation of a portion of the homo- 
geheous yolk against the inner surface of the vitelline sac. The 
mhode of origin of this vesicle is very important to notice, be- 
Cause it bears reference, in a very pointed manner, to the theory 
of the origin of free cells. Here it is evident that the egg-cell 
id not originate, as is usually stated of cells, around its nucleus, 
but that its nucleus is the offspring of the cell which encloses it. 
the Purkinjean vesicle always remains close to the parieties of - 
the ege, even to the time when it disappears, and in no way 
Points to a falsely claimed relation to fecundation and the origin 
9 the embryo. The wall of this vesicle originates like that of - 
me tlline sac, about a previously conglomerated mass of 
icles, 
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The appearance of numerous vesicular bodies, known as the 
Wagnerian vesicles, upon the inner surface of the wall of the 
Purkinjean vesicle, completes the operations, which have here 
been going on, necessary to the perfecting of the egg-cell, and 
the rendering of it, although a cell, totally different in properties 
m all other apparently similar cells. 
ee he development of the yolk.—It will be more convenient after 
on account of the complicity of the contents of the egg, to 
Peat of its different constituents separately. We will commence 
With the yolk first, as that is the fostering substance from which 
all the other components essentially originate. Previously to 
e yolk-cell nuclei which have undergone changes 
le ‘embryo,’ and they alone remain free, circulating 
ed out in a mass of cells identical with themselves. These 
ne the first cells originating interstitially, but yet, after all, not ers | 80, as is 
man *° With the egg; for each blood corpuscle is a segment of an original yolk-cell 
ets, which has gone through the process of self-division; whilst the egg originates 
of 8s the primary yolk-cell does, by conglomeration of particles, and the formation 
* membrane around the parieties of this concretion.” 
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