308 STUDIES IN GENEKAL 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 1 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. 1 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 r 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. 



1 Or rather, its power of absorbing liquid. [1903] 



