WATER 



strated by several experimental methods we may say that 

 slight experimental modifications of the protoplasm cause 

 movement of water out of the structures which hold it. 

 The conclusion is that water as an integral part of the col- 

 loid structure of protoplasm is in the internal phase. This 

 does not mean that no water exists otherwise in living 

 protoplasm. On the contrary, we can only account for 

 the observed initial shrinkage of an Qgg on return from 

 hypotonic to normal sea-water as due to the invisible escape 

 of water which doubtless represents, in part at least, water 

 held as in the external phase. 



Thus we have answered the first question raised: how is 

 water held in protoplasm 1 



I may point out that these experiments as such have 

 some significance for medicine. The distribution of yolk 

 differs with different eggs, for instance in that of Nereis 

 and Platynereis, as I have said above. Likewise, during 

 its development the yolk changes in location as has been 

 shown. Undoubtedly some of the difference in water- 

 holding capacity noted among eggs of several species or in 

 the same egg at different stages is to be attributed to this 

 difference with respect to yolk. What is true of yolk may 

 be true of other bodies in the cytoplasm. In diseased con- 

 ditions as oedema and nephritis in which tissues hold an 

 excessive amount of water, the structure of the cell may be 

 a very important factor in determining this abnormal water 

 content. 



I turn now to the consideration of the question of the 

 movement of water into and out of cells. This calls for a 

 statement concerning the nature of the cell-surface. 



When this question of the passage of water and dissolved 

 substances into and out of cells is raised, especially in physio- 

 logical work, by the term, cell-surface, usually is meant a 

 membrane. This membrane is then spoken of as semi-per- 

 meable because it is said to take up certain substances and 



