448 FRANK R. LILLIE 
But it is clear at least that the maturation of the egg does not 
differ from the maturation of the spermatozo6n in this respect. 
In both cases capacity for further cell-division is lost after the 
second maturation division, and itis quite natural, certainly, to 
postulate similar causes for this phenomenon in both sexes. If, 
as the results of the present study indicate, karyokinesis is the 
result of a certain qualitative nucleo-plasmic relation (to be dis- 
tinguished from R. Hertwig’s quantitative nucleo-plasmic rela- 
tion) we have to postulate in both cases a disturbance of this 
relation. And as this relation must be conceived as a chemical 
interaction of some kind, precipitated possibly by rhythmical 
changes of permeability of the nuclear membrane, the alteration 
in question must involve either the establishment of an imperme- 
able condition of the nuclear membrane or a chemical change in 
nucleus or cytoplasm. But we have seen in Nereis, that, even 
when the membrane of the nucleus of the mature egg breaks down, 
no karyokinetic phenomena follow, unless the egg is fertilized. 
So the phenomenon of cessation of division can hardly be con- 
ceived as conditioned by the membrane alone. 
The egg-cell and the spermatid are not the only cells that lose 
the capacity for division in the course of development. In the 
course of senescence all cells lose this capacity, and studies in 
cell-lineage have shown that certain cells entirely lose the capacity 
for division in very early stages. I need cite only the case of the 
so-called turret cells in Crepidula (Conklin ’97), which are formed 
in the sixteen cell stage, and which divide only twice during the 
cleavage period. Mead has called attention to similar cases in 
his studies of cell-lineage in Annelids (Mead ’98). Cessation of 
division cannot be a problem of centrosome or no centrosome in 
such cases; nor yet in the case of the spermatid. A much more 
profound physiological cause must be involved. 
Constructive metabolism has come essentially to a standstill 
in the mature gametes; the rate of metabolism in the mature un- 
fertilized egg as tested by oxygen consumption is many times less 
than that of the fertilized egg (Warburg, ’05). Child (11) cites, 
as conditions that lower the rate of metabolism, decrease in per- 
meability, increase in density, accumulation of relatively inactive 
