CELL-DIVISION 



at the surface of the egg. Though this susceptibility is 

 not of immediate concern here, since it does not relate to 

 the cleavage-cycle, we adduce it since it shows susceptibility 

 as due to ectoplasmic change. In the first period of sus- 

 ceptibility which occurs during the cleavage-cycle and 

 which falls in with the stage of union of the egg- and sperm- 

 nuclei, when there is a normal shrinkage of the egg-contents 

 and the ectoplasmic filaments are most easily visible, the 

 egg does not disrupt at definite points. In some other 

 eggs, those of Crepidula,^ for example, it seems that the 

 weakness that appears at this time or soon after, persists. 

 In the period prior to cleavage, there are localized points 

 of weakness due to ectoplasmic activity; they are limited to 

 the egg-poles. In all periods, susceptibility is due to 

 changes in the ectoplasm and where these are local, break- 

 down of the egg is likewise localized. 



In none of these three periods of susceptibility can the 

 break-down of the egg be ascribed to changes in surface- 

 tension. It is not possible to regard the complex structure 

 of the ectoplasm with its palisade arrangements of filaments 

 and their covering membrane as a simple surface-film. 

 The width of this covering membrane alone surpasses by 

 far that of a molecular film. Besides, this membrane is 

 passive in cell-division; ectoplasm and filaments play the 

 active role, changing from moment to moment first in one 

 region and then in another. These changes in structure 

 and behavior parallel the periods of susceptibility. 



The ectoplasm of marine eggs is a highly mobile, con- 

 stantly changing structure. In eggs as in other cells, 

 protozoan, nerve, muscle, etc., it is no mere inert sheath. 

 Instead, it reflects in its spinning, in its suceptibility, the 

 irritability of the living system in response to environmental 

 changes. In addition to the significance that its general 



1 Conklin, ig/2b. 



2S3 



