74 BIOLOGICAL LECTURES. 



by the direction of the contained spindle," but " the form of 

 the cell has no effect upon the position of the spindle " (p. 72). 

 Experiment has shown that the position of the spindle and 

 the direction and size of division may be modified by pressure, 

 and this again has lent support to the view that the position 

 of the spindle is the determining feature in cell division. 



b. Centrosome. Others consider the centrosome the prime 

 mover in cell division, " the dynamic centre of the cell." Rauber 

 ('83) holds that the position of the spindles is the result of the 

 mutual attractions of neighboring asters. Heidenhain ('95) re- 

 fers the direction of division to a definite angle of rotation of 

 the centrosomes. 



c. Cytoplasm. Boveri ('97) has shown that neither nucleus 

 nor centrosome can be said to take the initiative in cell division, 

 since the mitotic processes may go on independently in each, 

 and both he and Driesch ('98) have shown that the rhythm of 

 cleavage depends upon the egg cell and not upon the sperm, and 

 therefore, in all probabilities, upon the cytoplasm and not upon 

 the nucleus or centrosome. 



III. PROTOPLASMIC MOVEMENT AS A FACTOR. 



Almost all views as to the factors of differentiation regard 

 the cell as in a static condition ; very few consider it from the 

 kinetic standpoint, though the very names karyokinesis and 

 cytokinesis suggest movement as the fundamental fact of cell 

 division. 



In the maturation, fertilization, and cleavage of certain, gas- 

 teropods I have observed successive stages of a complex and 

 orderly movement of the entire cell body by which the positions 

 of the cytoplasm, yolk, nuclei, centrosomes, and mid-bodies 

 (Zwischenkorpern) are changed in a definite and orderly way. 

 Unfortunately, I have been unable to actually observe these 

 movements in the living egg, since the eggs studied contain a 

 large amount of yolk and are therefore opaque, and since the 

 movements described are very slow. However, the evidences 

 of these movements are so abundant and unmistakable that one 

 could not be more certain of them if he had seen the actual 



