CENTRIFUGAL FORCE ON EGGS OF CREPIDULA 363 



In these movements we may distinguish the active movements 

 of protoplasm which occur in the localization of cytoplasm, 

 nuclei and mitotic figures, and the passive movements of inclu- 

 sions such as oil, yolk and pigment. These passive movements 

 may be considered first since they are simpler than the active 

 ones. They are plainly of two kinds: 1) inclusions such as pig- 

 ment, granules, water, etc. may be carried along with proto- 

 plasm in its active movements, as in the case of the yellow pig- 

 ment in the 'mesoplasm' of Cynthia (Conklin '05); 2) by the 

 concentration of protoplasm in certain regions inclusions may 

 be forced out of those regions as in mitotic spindles and asters; 

 similarly the concentration of protoplasm at the animal pole 

 forces yolk to the vegetal pole. Such passive movements are 

 due to active movements of the protoplasm and require no 

 further explanation. 



The active movements of protoplasm are more difficult to ob- 

 serve and explain. The denser portion of the protoplasm is 

 highly elastic and contractile, as Kite has shown by direct ob- 

 servation, and its capacity for movement is probably due to this 

 property. Thus the flowing of peripheral protoplasm to the 

 point of entrance of a spermatozoon and the formation there 

 of an entrance cone may be regarded as due to the contraction or 

 concentration of this protoplasm to the point stimulated. Prob- 

 ably the collection of spongioplasm around the sperm centrosome 

 or in the aster of any mitosis is likewise due to the contractility 

 of this substance. The movement of the sperm nucleus and aster 

 toward the animal pole and the segregation there of most of the 

 spongioplasm may be explained in the same way. If the 

 spongioplasm is highly contractile in all directions and concen- 

 trates to a point of stimulation these and many other cell activi- 

 ties find a ready explanation. The possibility of such concen- 

 tration depends of course upon the fact that spongioplasm is not 

 uniformly distributed throughout the cell but that it exists in- 

 termingled with other substances and that in its concentration 

 to one point these other substances are displaced. In former 

 papers (Conklin '02, '12) I have dealt at some length with the 

 movements of metakinesis and telokinesis and since I have 



