THE BIOLOGY OF THE CELL SURFACE 



that always follows a definite path or along preformed 

 channels; these movements represent specialized conditions. 

 Many cells, especially egg-cells, exhibit streaming which 

 is rhythmical but follows no preformed channels. This 

 streaming can be observed in egg-cells particularly if they 

 contain easily visible formed bodies or if having been 

 bathed in a solution containing a non-toxic dye they took up 

 particles whose movements can be easily followed. 



The best of the earliest descriptions of this type of move- 

 ment in cytoplasm is that by Goette.^ Since his time 

 many workers, among them von Erlanger,^ have studied 

 the phenomenon. In living eggs one can of course fol- 

 low these changes from minute to minute. So-called 

 resting eggs — i.e., those that remain in a given stage, as for 

 instance mature but unfertilized eggs — do not show this 

 type of movement. If such mature resting eggs be fer- 

 tilized, then the cytoplasm begins to stream. During each 

 cleavage-cycle there is a period of little movement and a 

 period of intense movement; these periods appear to parallel 

 the stages in the cycle of nuclear changes. 



With the growth of the asters these movements become 

 most marked. They begin over the spindle poles and move 

 toward the spindle equator. As the opposing streams meet, 

 they have only one possible direction: they can not move 

 outward because of the resisting surface and thus move 

 towards the centre of the egg. In this way the curreints 

 foreshadow the future cleavage plane; before the actual 

 division of the cell-body the currents have brought about 

 that disposition of inclusions found when the plane forms. ^ 



Before fertilization the inclusions maintain constant posi- 

 tions without reference to their specific gravity. If in such 

 eggs they are segregated by means of centrifugal force, 



1 Goette, i8js. 



'- Von Erlanger, iSgj. 



^ Cf. Goette, iS/j, loc. cit. 



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