A Comparative Study of tlie Structure and Origin of tlie Yolk Nucleus. 675 
reaction of chromatin, which is prominent in tlie nuclear reticulum of 
these eggs. 
On coagiilation, the karyolymph which has cntercd the aster and 
formed its matrix, obscures the delicate fibers constituting the frarne- 
work of the aster. Only in rare cases c-an the centrosome itself bc seen 
in the center of this coagulated mass. 
There are, therefore, in the cytoplasm of the eggs of the tortoise, 
Clemmys marmorata, three parts which wonld be called yolk nucleus if 
seen alone: 1. the centrosphere ; 2. extruded karyolymph, filling the meslies 
of the centrosphere; 3. the metaplasm. 
The first or centrosphere is a continuation of the centrosome of the 
dividing oogonia. The karyolymph is extruded from the nucleus, where 
I have elsewhere suggested, it is produc-ed in or by the chromatin. The 
metaplasm is a new Chemical compound arising in the cytoplasm through 
the action of the karyolymph on some substance in the cytoplasm. It 
may be a synthetic food product which is gradually absorbed in the growth 
of the egg. The capacity of the egg cell for growth as eompared with the 
follide cells, may be due to the persistence of the centrosome in the former. 
Whether my inference from appearances be correct or not in this 
important matter, there is no reason why we continue longer to regard 
everything differing from the rest of tlie cytoplasm as one body, to be 
designated by one general term, yolk nucleus. 
Before proceeding to the description of similar tliings in other eggs, 
I would like to suggest that tlie ident ification of a part of the socalled 
yolk nucleus as the centrosome or centrosphere, ouglit not to be considered 
a radical assumption at the present time. The Nebenkern of sperm cells, 
more often tlie middle piece, has been declared to be the centrosome on 
far less adequate evidence than that which these eggs present. And 
on apriori grounds, the persistence of the centrosome in the cytoplasm 
of the oocyte ouglit not to be regarded with greater scepticism than an 
affirmation of its persistence in sperm cells. The fact may be alter all, 
that there is yet much to be learned about the centrosome. The yolk 
nucleus may be the body which because of its greater size, may add to 
our knowledge of that minute dot usually taken to be the centrosome in 
its typica-1 form. 
What I have called the cytocenter, tlie centrosphere, has a constant 
relation to the nucleus and to the cytoplasm. It becomes the center of 
the vegetative pole of the egg; and together with the germinal vesicle, 
locates the egg axis: and consequently is involved in the determination 
of the two primary germ layers. 
