Limits oF DivistBpiutty or Living Matter 325 
more than two embryos from an egg. If the eggs are made 
to burst before segmentation begins, only one nucleus is 
present, and in consequence either the contents of the egg 
or the extraovate must be without a nucleus. I have already 
mentioned in my earlier papers that in the course of the 
segmentation nuclear material gets into that portion of the 
protoplasm which was originally free from it. Occasionally 
the extraovate is cut off before nuclear division occurs. 
Nevertheless, cleavage occurs. From the observations of O. 
and R. Hertwig and of Boveri we may assume that in these 
cases a spermatozoon has entered the protoplasm. The 
nuclear material which is introduced in this way suffices to 
inaugurate the process of cleavage. 
3. In these experiments it is natural, of course, that the 
extraovate, as a rule, does not contain exactly one-half the 
mass of the egg. Those cases in which the extraovate and 
the contents of the egg differ greatly in size are well adapted 
to decide how large an amount of the egg-substance is just 
sufficient to give rise to a normal pluteus. I followed the 
development of selected individual eggs with their extra- 
ovates in a drop of water contained in a moist chamber. I 
also examined very carefully from day to day cultures of 
such eggs kept in large vessels, and determined the size of 
the smallest plutei. Finally, I studied also from day to day 
the fate of these small fragments. The results of these ob- 
servations, which I pursued uninterruptedly for two months 
last year and again for two months this year, were very defi- 
nite, and may be expressed as follows: 
a) The smallest normal plutei which arose from fragments 
of an egg were linearly about half the size of the plutei aris- 
ing from a whole egg of the same culture. Their volume— 
their density being considered the same— was therefore about 
one-eighth of that of a normal pluteus. 
b) Smaller fragments of an egg than these developed into 
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