308 STUDIES IN GENERAL PHYSIOLOGY 
processes in the formation of twins. For I have found that 
from the moment of the entrance of the spermatozoon into 
the egg the osmotic pressure’ of the egg increases greatly. 
If unfertilized eggs are introduced into dilute sea-water, 
their volume increases relatively little. As soon as the sper- 
matozoon enters the egg, however, or when an egg which 
has just been fertilized is brought into the same salt solution, 
its volume increases very markedly, as I have determined by 
actual measurements. This fact shows that the spermatozoon 
brings about chemical changes in the egg which cause an 
increase in its osmotic pressure.’ I will return to the dis- 
cussion of this point in my more complete description of 
these experiments. 
So far as osmotic pressure is concerned, great differences 
exist between the eggs from one and the same individual. 
Even when the sea-water was only slightly diluted, a small 
percentage of the sea-urchin eggs burst; and I do not doubt 
that this may occasionally happen in normal sea-water. 
Whatever may be the actual process in the formation of 
twins from one egg in mammals, it seems probable that all 
multiple formations from one egg are caused primarily 
through complete or partial mechanical, or at least physical, 
division and isolation of the substances of the egg. 
10r rather, its power of absorbing liquid. [1903] 
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