456 EMBRYOLOGY OF THE TURTLE. Part III. 
still more differentiated, stands out as a tenacious layer, capable of holding in 
durance the internal, fluid, mobile portion of the conglomeration. 
Anterior to this, in some cases, (Pl. 8, fig. 1, ¢, c',) but not always, a nucleus, the 
germinal vesicle, makes its appearance. There is, however, no relation between the 
size of the egg and the period of the first appearance of the germinal vesicle; at 
one time we find it nearly filling a small ege (fig. 1, ¢, c!); at another, it is not 
at all present in a comparatively large one (PI. 8, fig. 1, f, 4%). Beyond a certain 
size, (Pl. 8, fig. 1, 4) however, about 5,45 of an inch in diameter, it is never absent ; 
so that there is a limit, on one side, to the irregularity of its development. It 
is hardly necessary, after what has been said, to remark, that the nucleus has 
no part whatever in the formation of the egg cell, but is entirely a subsequent feat- 
ure of the contents among which it is imtroduced. True enough, there is a solid 
substance around which its cell wall arranges itself, just as most observers have of 
late advocated, but here the parallel ceases; for the basis of this operation is 
not the nucleus, the germinal vesicle, as they would have it, but the yolk, to 
all intents and purposes.| The nucleus, in this instance at least, is often a feature 
of very tardy appearance, and always arises like a swelling, having the concave 
contour of the ege-cell wall for a basis. It is always very transparent, but most 
especially so im the younger stages of the growth of the egg, (Pl. 8, fig. 1, m, n, 0, p, 
fig. 3 and 3a,) when it is often difficult to determine its outline definitely, its trans- 
parency arising from the mode of its origin, of which we will speak presently. 
The germinal vesicle is always visible to the naked eye, in eggs of from 
one sixteenth of an inch in diameter to full-grown ones; its presence is indicated 
by a clear, dark, round spot at the surface, where it origimates, as will be shown 
below (PI. 9, fig. 9 and 10; and Pl. 9a, fig. 32 (7) and 32a). 
on the distal side of the egg, and at other times next to the attachment of the ovum? 
It appears very often 
1 Barry, (Phil. Trans., London, 1838, p. 808-310, 
pl. v., fig. 1, 18, and 19,) Leuckart, (Handwérterbuch 
der Physiologie, ete., von Dr. Rudolph Wagner, arti- 
cle Zeugung, p. 815,) and Thompson, (Cyclopedia of 
Anatomy, London, 1854, article Ovum, p. 76 and 77, 
fig. 53,) all hold to the origin of the yolk substance 
around the previously existing germinal vesicle ; but 
if we mistake not, in many instances the germinal ves- 
icles of these authors were the true eggs, and in others 
the vitelline sac was concealed by its close contact 
with the wall of the Graffian follicle. 
2 The small germinal vesicle of this magnified egg 
will be perceived on the left side of the figure, about 
one third from the centre. 
§ Various authors, who haye made more or less 
special investigations upon this subject, assert that the 
germinal vesicle, throughout the animal kingdom, has 
a central position in the younger stages of the egg, 
but that later it approaches, and finally plants itself 
at the surface. Now, since we have pointed out its 
superficial position from the very beginning, in Testu- 
dinata, and have observed the same feature in the 
eggs of other animals, as future volumes will show, 
we have reason to believe that the assertion of these 
authors, respecting the situation of this vesicle, can- 
not be sustained by more rigid researches. Such a 
situation may be only apparent, owing to the position 
of the egg under the microscope. 
