ZOOLOGY: E, a CONKLIN 
89 
substance' remains unaltered. It is evident from Lillie's use of this 
term that he means it to include what is commonly called ^protoplasm' 
as contrasted with the ^inclusions' or metaplasm. 
In eggs which contain relatively little yolk it may appear possible 
to identify this 'ground substance' with the general protoplasm, but 
in the eggs of Crepidula, where the yolk constitutes more than three- 
quarters of the whole, the protoplasm, including under this term cyto- 
plasm, nucleus and centrosphere, is moved bodily from one pole to 
another when the yolk is displaced from its normal position. If there 
is a 'ground substance' here which is not moved by centrifuging it must 
constitute a relatively small part of the general protoplasm of the egg. 
There is good evidence that this is indeed the case and that while the 
greater part of the cytoplasm is more or less free to move under the 
influence of centrifugal force there is a portion which is denser and more 
resistent which is not stratified by centrifugal force. Indeed as long 
as an egg remains aHve this stratification is never complete and the 
boundaries of the zones are neither plane surfaces nor are they sharply 
delimited as is the case in dead eggs which have been centrifuged. 
In Crepidula this denser and more resistant part of the protoplasm is 
found in the nucleus and centrosphere, in a thin peripheral layer which 
retains its position as long as the egg remains unbroken, and in a frame- 
work of strands which runs through the cell and connects the nucleus 
and centrosphere to the peripheral layer. During non-divisional stages 
these strands are slender and less resistant and may be stretched or dis- 
torted but are rarely broken. They hold the centrosphere and nucleus 
in a definite relation to each other and to the cell axis, the centrosphere 
always lying on the side of the nucleus toward the animal pole and next 
to the free border of the cell. In whatever axis the egg may be centri- 
fuged and however far the centrosphere and nucleus may be carried 
from the animal pole, the centrosphere always retains this position on 
the animal pole side of the nucleus and often can be seen to be con- 
nected with the peripheral layer by strands of denser protoplasm. 
During cell division the strands radiating from the centrosome are 
stronger and more resistant than during non-divisional stages so that 
it is much more difiicult to move the mitotic figure than it is to shift the 
resting nucleus and centrosphere. Indeed it is practically impossible 
to move this figure when once the astral radiations are well developed 
without destroying the power of further development. On the other 
hand if nucleus and centrosphere are displaced from their normal posi- 
tions during non-divisional stages and are prevented from returning 
by prolonged centrifuging they undergo division in these new positions 
