STUDIES IN ARTIFICIAL PARTHENOGENESIS. 153 



brane behaves toward reagents in the same manner as does the 

 membrane on egg fragments, we are justified in regarding it too 

 as a precipitation membrane. The most noteworthy feature of 

 the precipitation membranes of Traube and Quincke is their 

 semipermeable (or partially semi-permeable) 1 character. \Ye 

 should therefore expect the vitelline membrane to exhibit semi- 

 permeable properties, and in this way to govern osmotic inter- 

 change. When the egg is caused to shrink in a hypertonic 

 solution, the vitelline membrane shrinks with it. This has been 

 observed in a great variety of hypertonic solutions. For such 

 a shrinkage, there are 3 possible types of explanation. The 

 inward tension may be the result of a force arising from (i) the 

 substances within the membrane, (2) the membrane itself, or 

 (3) something immediately outside of the membrane. 



Physico-chemically the possibilities, as I see them, are (i) 

 The fluid interior of the egg is coagulated by the various hyper- 

 tonic solutions, shrinks as a result and pulls the membrane with 

 it. (2) The vitelline membrane as a result of its semipermea- 

 bility 2 is responsible for the shrinkage. (3) There is an invisible 

 semipermeable membrane outside of the vitelline membrane. 

 As there is nothing to warrant the assumption of any semiper- 

 meable membrane outside of the vitelline membrane, it seems 

 scarcely necessary to discuss the third possibility. The first 

 possibility is essentially the position maintained by M. Fischer 

 ('io). He regards the passage of water into and out of cells as 

 due primarily to the attraction of the interior colloids for water. 

 According to this view endosmosis, such as occurs in hypotonic 

 solutions, would be the result of the taking up of water by col- 

 loids within the cell. That the process of water absorption is 

 by no means dependent on the affinity of the egg colloids for 

 water is conclusively proven by the fact that influx of water 



1 The existence of an absolutely semipermeable precipitation membrane (i. e., 

 one which prevents the passage of all substances in solution, but permits the 

 passage of solvent) is extremely doubtful; cf. for example Quincke '02. The fact 

 that a substance may penetrate a membrane and yet exert considerable osmotic 

 pressure against it, has often been neglected by biologists. To assume that 

 because a substance passes through a membrane it can exert no osmotic pressure 

 against it, is just as foolish as to assume that the air in an air-bubble exerts no 

 pressure on the film of water surrounding it. 



- Using the term in its broadest sense. 



