Limits oF DivisiprLity oF Living Matter 337 
one-eighth of the mass of the entire egg (nucleus plus pro- 
toplasm). 
c) The amount of substance necessary for the formation 
of a blastula is much smaller than that necessary for the 
formation of a pluteus; for the formation of a gastrula more 
substance is probably required than for the formation of a 
blastula. 
d) It does not matter which position the fragments of an 
egg of Arbacia occupied in the intact egg; so far as divisi- 
bility is concerned, the protoplasm of the Arbacia egg can 
certainly be considered as isotropic. 
e) Since the limits of divisibility are almost the same in 
the unsegmented egg as in the first stages of segmentation 
(the thirty-two-cell stage included), it follows that (a) no 
qualitative changes occur in the egg during the early stages 
of segmentation which restrict the development of fragments 
of the egg, and that (8) the individual cleavage cells, so far 
as the limits of divisibility of the substance of the egg are 
concerned, may be considered as equal. (In other respects, 
however, differences may exist between the individual cleav- 
age cells.) 
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