A Comparative Study of the Structure and Origin of the Yolk Nucleus. 685 
channel along whieh the karyolymph from the nucleus is conveyed to 
the expanding centrosome and sphere. 
The constancy of the yolk nucleus in these eggs, precludes the Inter- 
pretation that it is a transient feature, an aecidental condensation, or a 
fortuitous concourse of atoms and molecules in an amorphous substance. 
There is a structure in the cytoplasra of eggs, revealed by the microscope, 
which enables one to recognize relationships between eggs of different 
classes of animals. 
As in the case of the tortoise, the spider and the kingcrab, the yolk 
nucleus (centrosphere) determines the vegetative pole of this egg; and 
together witli the germinal vesicle, with which it Stands in the most in- 
tiraate relation, it determines the egg axis. Through this body, therefore, 
there is an unbroken succession from the primitive germplasm, handed 
down from the preceding generation to the development of the primary 
germ layers, from which all organs finally develop. 
The Frog’s Egg. 
I have not studied the frog’s egg very carefully with reference to the 
presence or absence of the yolk nucleus in the very young egg. But I 
have found indications of a body resembling that in the pigeon’s egg. It 
tends to form a ring around the germinal vesicle somewhat like that 
seen in the kingcrab, fig. 38. In this ring are irregulär, stainable masses, 
between the inner and outer zone resembling the metaplasm in the egg 
of Clemmys. 
At one pole, there is an aggregation of such granules both outside 
the germinal vesicle and inside. Many of them look like deeply staining 
nucleoli. That these nucleoli come out bodily from the germinal vesicle, 
I hesitate to believe. 
The Cat’s Egg. 
In favorable preparations of the ovary of the cat, eggs can be found 
showing a yolk nucleus (centrosphere) plainly. Such an one is represented 
in fig. 56. These are beautiful preparations, with no indication of shrink- 
age inside or outside the egg. The germinal vesicle is uniformly spherical, 
and contains a nuclear network of chromatin in which there is a com- 
paratively large spherical, and deeply staining nucleolus. At one pole of 
this germinal vesicle, there can be seen by proper manipulation of the 
illuminating apparatus of the microscope, a round area, nearly as large as 
the nucleus, and slightly more translucent than the rest of the cytoplasm. 
Careful focusing shows a ring of granules in the center, and a little granule 
45 * 
