Cuap. I. DEVELOPMENT OF THE YOLK CELLS. 467 
single mass, which but faintly shadows forth the parietes of its constituents (Pl. 
9,. fig, JAB Nan G 11g): 
Formation of the Mesoblast. We have already mentioned, that there is no con- 
stant size at which the ectoblast develops its mesoblast. A minute ectoblast may 
be seen with a well defined mesoblast, (PI. 8, fig. 22, ¢, d, and Pl. 9, fig. 8a, 
F, 2,) and another four times its diameter (Pl. 8, fig. 21a) without any, and so on, 
at various intermediate magnitudes. Some cells have even their entoblasts (Pl. 8, 
fig. 23b, a, b, 23d, d, e; Pl. 9, fig. 8a, D, BE, F) at a diameter, which, in others, 
presents only a homogeneous content. However, to give, with some sort of pre- 
cision, an idea of the limit of the development of the mesoblast, we would state, 
that, under a magnifying power of five hundred diameters, there cannot be found 
a single cell, except perhaps with rare exceptions, which presents to the eye a 
diameter beyond one eighth of an inch, that is, zj459 of an inch im actual size, 
without a mesoblast. 
As a natural consequence of this diversity of size in relation to the appear- 
ance of the mesoblast, this body may be much larger in one cell, than in ano- 
ther of the same diameter; for as soon as it is defined it begins to grow, and 
continues to increase in size, along with the ectoblast, till the egg drops from 
the ovary to enter upon a new phase of life. Unlike other mesoblasts, the 
mesoblast of the yolk cell is not arrested in its development when the ectoblast 
has reached a certain size, and does not remain as a mere indication of past 
activity, but ever persists in manifesting very active internal changes within its 
constantly increasing bulk. With the exception of the germinal vesicle, the meso- 
blast of the yolk cell, when at its full development, is the largest known, at 
times measuring about ;4, of an inch in diameter. With these preliminary 
remarks, one may be forewarned to expect here a mode of development of meso- 
blasts hitherto unnoticed or disregarded in other centres of cell evolution. 
The earliest indication of a mesoblast is manifested by a slight haziness at 
one single point within the ectoblast, close against its wall (Pl. 9, fig. 8a, J, 
L!, 4). At first undefined and vanishing at its border, it gradually assumes a sharp, 
spherical outline and a pearly opacity reminding one of the primary physiognomy 
of the ectoblast; but yet it is of a denser nature than the latter. The size at 
which it gains its definiteness of contour varies in different cases, the hazy state 
of one often exceeding the clearly limited mass of another by several diameters. 
The attachment to the wall of the ectoblast is at times loosened almost as 
soon as the outline is perfected (fig. 8a, J); however, not long afterwards, every 
mesoblast becomes free, and may be found, for the rest of the interovarian life 
of the ege, at the centre of the sphere, whence it is derived. 
In the case of the formation of a mesoblast in a most minute ectoblast, the 
