Cuap. I. DEVELOPMENT OF THE YOLK. 461 
Having thus sketched beforehand, as it were, and described, the grosser changes 
in the yolk mass up to its maturity, we will now return to the starting point of 
this digression, and indicate the intimate structural changes which the yolk cells 
undergo successively, as these changes correspond to each successive feature of 
the growing egg. 
At the time the granules begin to invade the clear space, (Pl. 8, fig. 6, 6a,) 
they are rather coarse and irregular in outline; but the next step beyond this (PI. 
8, fig. 8a) in the growth of the egg reveals a diminution in their size, as if 
they were, as is probable, redissolved by their mixture with the more albuminous 
fluid which has received them. Soon afterward, in an ege not much larger than 
the last, or even of the same size, they again appear very coarse, yet dark and 
irregular, and withal lighted up by seven or eight quite large, clear, albuminous 
globules, scattered irregularly in different parts of the mass (PL 8, fig. 7). These 
globules, as we have seen above, are the remains of the hyaline region of the 
younger egg. That they are not oil drops, such as have been described by vari- 
ous authors as occurring in certain stages of the growth of the egg, is easily 
demonstrated, first, by their very faint refraction, (Pl. 8, fig. 7, and 16a, 16b,) and 
most conclusively by their mode of origin, as already described. The slightest 
pressure diffuses them through the yolk mass, whilst oil globules are more tena- 
cious, and if they break up, each fraction at once assumes a globular form. 
Another slight advance, in eggs of about 54, of an inch in diameter, again 
brings before us a finely granulated yolk, pretty evenly distributed throughout the 
eg. A still finer granulation, almost imperceptible, occurs throughout another ege 
which is hardly larger (PI. 8, fig. lla). The application of the extreme high 
powers of the microscope, however, shows that these granules are spheres of dark, 
oily globules (fig. lla, a) closely packed together, which would be perfectly invis- 
ible under an amplification of four hundred diameters, and leave one to suppose 
that nothmg but a homogeneous fluid occupied the field. Other eggs, (Pl. 8, fig. 
9, 9a,) of the same size as the last, are far from resembling it: hardly one half 
of the yolk is dense and dark, and amid the finer materials, coarse angular grains 
arranged in heaps are scattered pretty freely, but as yet few grains appear in 
each heap; within the lighter space these grains are much less numerous, being 
only grouped in twos or threes, and even that not frequently. In this portion of 
the yolk are also very numerous minute but distinct particles, like dust floating 
im the air across a sunbeam. The clear globular spaces previously mentioned are 
defined by the anastomozing of these heaps, which form irregular, sponge-like meshes. 
A further approach of these heaps to each other is observed in more advanced 
stages of growth of the ege; but, before considering these changes, we must not 
pass over an intermediate condition of peculiar features, which has seldom been 
seen during our investigations. 
