36 EDWIN G. CONKLIN 
There are no eggs wholly without yolk and probably in all of 
them plasma is formed at the expense of yolk during the cleavage 
period. This probability is of great significance, for all studies 
which have had to do with the relative quantities of protoplasmic 
and nuclear materials during these early stages of development 
have dealt only with the entire cell contents without attempting 
to determine what part of this is plasma. In many cases, the 
great disproportion between cell volume and nuclear volume at 
the beginning of developemnt is due to the fact that a large part 
of the cell volume is made up of yolk; if the volume of the plasma 
only is compared with that of the nucleus it is found that the 
relative quantity of plasma is actually less at the beginning of 
development, than in the later cleavages, with the single excep- 
tion of those blastomeres which have unusually long resting 
periods. In Crepidula there is no excess of plasma over nuclear 
material in the early stages, in comparison with the later ones, 
as Minot and others have assumed, and the process of cleavage 
is not in this case a method of restoring the Kernplasma-Norm, 
or of rejuvenating senile cells, by an enormous increase of nuclear 
material as compared with the plasma. As a matter of fact 
the plasma increases almost as rapidly as the nuclear material 
during the cleavage of this egg, and even adult tissue cells have 
a Kernplasma-Relation but little different from that of the 
blastomeres, (see p. 25). 
VI. Rate of nuclear growth during cleavage 
It is well known that during cleavage there is usually no in- 
crease in the volume of the egg, but it is generally held that the 
increase in the nuclear substance is very great. In his book 
on ‘‘Age, Growth and Death’ Minot (’08) says: ‘The nuclei 
multiply (in cleavage); they multiply at the expense of the pro- 
toplasm. They take food from the material which is stored up 
in the ovum, nourish themselves by it, grow and multiply until 
they become the dominant part in the structure” (p. 166). He 
suggests that this nuclear increase during cleavage is a process of 
rejuvenation, though he admits that the relative increase of nu- 
