THE BIOLOGY OF THE CELL SURFACE 



reaction to environmental influences rhythmical changes of 

 resistance and susceptibility that are both difi"erent from 

 those shown by the unfertilized egg. 



The new egg sea-water equilibrium is established by the 

 structural changes in the ectoplasm; these changes do not 

 reverse themselves and the egg does not return to the 

 physiological state it had previous to treatment or to fer- 

 tilization. Also with respect to water-movements these 

 same changes sharply set off the devel ping egg from the 

 untreated and unfertilized. Since the egg loses and gains 

 water concomitantly with each cleavage-cycle, one can not 

 say that with break-down of substance in the ectoplasm it 

 becomes permanently dehydrated. Rather, a momentary 

 water-loss as a consequence of the stimulus of inducing 

 means or of the spermatozoon, brings about a change to a 

 new level with respect to equilibrium with the surrounding 

 medium, and on this new level the ensuing rhythmical 

 process of water-entrance and -exit which accompanies the 

 developmental process takes place. 



The rhythmical movement of water, during the cleavage- 

 cycle, into and out of the egg undoubtedly means a move- 

 ment of water from place to place within the egg; and this 

 in turn means local and temporary hydrations and dehydra- 

 tions. These redistributions of water even in minute intra- 

 cellular dimensions are favorable for reactions.^ One 

 visible structure of the cell which in addition to the ecto- 

 plasm exhibits rhythmical changes during the cleavage- 

 cycle is the nucleus; its break-down and reformation is 

 indeed the criterion used for defining the cleavage-cycle. 

 As we have seen in the chapter on the fertilization-process, 

 the establishment of the mitotic complex constitutes the 

 index of the completion of the initial stage of development. 

 And I suggest that in all modes of initiating development, 



^ Just, 1937b. 



244 



