A Comparative Study of the Structure and Origin of the Yolk Nucleus. 673 
metaplasm; and I take it to be the resnlt of karyolymph united to, or 
acting upon food matter in the cytoplasm, that has not yet been assimilated. 
This substance accumulates around the centrosome, giving rise to 
a large body at one pole of the germinal vesicle, and occupying a space 
about the size of the germinal vesicle, figs. 2, 5. I presume this cor- 
responds to the latebra of the bird’s egg. 
There are evidences of concentric circles around the centrosome, 
having a definite relation to the germinal vesicle. There seems to be two 
different substances — one takiug the stain much more readily and deeply 
than the other. The latter generally occupies the center, while the former 
is peripheral, fig. 6. 
The less stainable portion is very finely granulär, in preserved ma- 
terial; and I incline to the belief that in the living egg it is fluid, filling 
the spaces between the fibers of the astral rays, and coagulated by reagents 
into a finely granulär precipitate. It may be that the two substances 
correspond to the white and the yellow yolk in bird’s eggs. 
The more stainable substance is either coarsely granulär, or eise 
appears as an amorphous sticky mass, surrounding the large central body 
as an irregulär ring, figs. 5, 6; or eise diffused, in isolated patches of 
varying size and shape throughout the cytoplasm, figs. 3, 9. 
The large, feebly staining body often seems homogeneous, and may 
assume gigantic proportions in comparison with the rest of the cytoplasm 
and the germinal vesicle. Its form may be circular in section, or oval, 
but occasionally greatly elongated and irregulär. But it always retains 
its connection with the nucleus, and it always occupies approximately 
the geometrical center of the egg. The germinal vesicle is consequently 
excentric: and being a constant feature, more or less conspicuous, it 
confers on the egg a distinct polarity, much as the latebra does in the 
hen’s egg. 
In many cases, perhaps in most cases, it is possible to make out a 
central, Condensed, spherical body as in fig. 8, the centrosome; and often, 
also, a series of concentric rings around this, fig. 2, suggesting the zones 
of yellow and white yolk in the hen’s egg. I take these zones to correspond 
to the concentric circles seen in the centrosome and aster of karyokinesis, 
and in leucocytes, pl. XXXIV, figs. 64, 68, 71, 74. This is seen in fig. 2 
when the large, central body is surrounded by an other wide zone of the 
deeply staining substance in which there are several spherical vacuolcs, 
seeming to be filled with a substance resembling the central body. 
In fig. 5 is represented, near the center of the large mass, a circular 
opening containing a System of fibers like a nucleus, reminding one of 
