A Comparative Study of the Structure and Origin of the Yolk Nucleus. 677 
OccasionaUy there seems to be a definite and comparatively thick 
limiting membrane, surrounding the body, as in fig. 19. But more com- 
raonly the concentric fibers pass impereeptibly over into the surrounding 
reticulum. In fig. 22, these fibers are seen to be massed at one side of the 
germinal vesicle, g. v. 
The outer zone of concentric fibers may form a layer of rather 
uniform thickness enclosing the central granulär portion, fig. 4. Occa- 
sionally a granulär layer has developed in the center of the ring, fig. 18. 
The central vesicle appears typically in figs. 15, 20, 22, 23, 24. Its 
similarity to a young germinal vesicle or to an ordinary nucleus is very 
striking. It is not at all surprising that early observers took it to be a 
real nucleus, as they often called it. 
As this yolk nucleus originates from a typical centrosphere of the 
oogonium, fig. 21, and as it retains these characters for some time after 
the young oocyte has begun to grow, fig. 11, and as it is always present 
and never more than one in each egg, I take all the later forms to be modi- 
fied centrospheres. The central granule in fig. 24 is, therefore, not a 
nucleolus as has been affirmed; but more probably a centrosome. 
The different appearances of the concentric layers is due to difference 
in condensation, or rather to differences in expansion of the network, 
which needs only an accumulation of cytolymph and metaplasmic gramdes 
to be just like the rest of the cytoplasm. The fibers are there, and the 
rneshes between them are there. Both are smaller, or let us say, less de- 
veloped than in the rest of the cytoplasm. The fibers of the yolk nucleus 
are continuous with the fibers of the cytoreticulum, fig. 23. In fact it 
is a part of the cytoreticulum which has not yet expanded. 
I venture to say, as I have said before, that the cytoplasm grows 
from this by a process of expansion. 
Morphologically, this yolk nucleus bears the same relation to the 
cytoplasm as the chromosomes bear to the nucleus. Given chromosomes, 
and a nucleus develops by the formation of karyolymph in the vacuoles 
of the chromatin substance, which thus is made to assume the form of 
the nuclear reticulum. Similarly, given a yolk nucleus like that of the 
spider, and cytoplasm may be formed from it by the formation of vacuoles, 
fig. 19, and a consequent expansion due to the mechanical pressure of an 
ever increasing cytolymph. Appearances suggest that the fibers also 
actually grow by intussusception both in thickness and in length. 
There is, in other words a typical centrosphere forming the frame- 
work of this body; and, as has already been noted, the karyolymph batliing 
this body as it comes out from the nucleus, forms one of the constituents 
