442 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
The Uebenkern has, among other things, been supposed to be homologous with the 
polar globules. I consider it the homologue of the yolk nucleus. It, like the yolk 
nucleus, arises from the nucleus not by any cell division, although closely associated 
with it. However we may homologize the reducing division in the male and in the 
female, or spermatozoon and egg, it is clear that in both cases the Nebenkern arises 
from the nucleus after the sex cell has assumed its final role of egg or spermatozoon. 
I wish here to point out the close resemblance of the yolk nucleus in Cymatogaster 
to the macronucleus of ciliate infusoria. Plate xcm will make the resemblance 
clearer. The resemblance of the different processes of conjugation of these protozoa 
to the processes of maturation of fertilization in metazoa has been pointed out by 
others, but I wish to carry this resemblance several steps further. It will be of 
advantage to give a brief review of the results which have been obtained recently 
in the study of the conjugation of ciliate infusoria. For plate xcm and its explana- 
tion, see page 446. 
The infusoria contain two nuclei : 
First, a large macronucleus which presides over nutrition and growth, repairs 
injury, and by its division enables the protozoon to multiply for a certain number of 
generations. If the micronucleus disappears through senescence, the macronucleus 
may still divide and enable the protozoon to multiply for a certain length of time, 
but afterward it loses this power and the individual containing it perishes. Its 
functions are all ontogenetic, of use to the race only in so far as they are of use to 
the individual. It may be looked upon as the somatic portion of the nucleoplasm 
of the protozoon, which, by simple divisions, builds a large number of individuals 
which collectively are comparable to the metazoon soma. It divides directly. 
Second, a smaller micronucleus, which divides indirectly, but whose division 
and indeed whose presence is not essential to the life and multiplication of the 
individual. It presides over the preservation of the race, and its function must come 
into play during a time corresponding^ to the period of maturity of metazoa. If it 
does not come into play at this time, it disintegrates and the individuals containing 
it are doomed to ultimate destruction. If it does come into play, the individuals 
containing it may continue to divide. It is of use to the individual only through its 
use to the race. It is of no direct use to the individual containing it. Its functions 
are all phylogenetic. 
In short, the macronucleus under any and all conditions is doomed to-destruction ; 
the micro nucleus may live forever under favorable circumstances. 
The macronucleus is dissolved at the time of or shortly after conjugation. A new 
one is formed from the segmentation nucleus. 
W e are now prepared to observe the similarities and the differences between these 
conditions and those obtaining in metazoa. 
The segmentation nucleus of metazoa contains, as in the infusorian, both micro and 
macro nuclear elements, but these are retained in varying proportions in its descend- 
ants, i. e., in the cells of the adult organism. Through a process of division of labor 
the power of rejuvenescence becomes restricted to comparatively few of the cells 
derived from the segmentation nucleus. The fate of all the remaining cells is final 
death. 
