CELL DIVISION IN EGGS OF CREPIDULA. 517 



correspond to the "ground substance" of Lillie (1906), but it is to be noted that 

 I do not regard these threads as the seat of polarity and organization, as I under- 

 stand Lillie to do, but merely as a framework which preserves the typical axial 

 relations of nucleus, centrosphere and cortical layer. But it is insufficient to say 

 that the organization of the egg determines the position of the mitotic figures 

 and consequently the cleavage pattern. More specifically the central spindle, or 

 netrum, usually forms at right angles to the cell axis (the axis passing through 

 the centrosome, nucleus and mid-body) and in the plane which separates the 

 nuclear halves or gonomeres. Owing to the movements of telokinesis in the 

 cleavage of Crepidula the cell axes undergo regular changes with respect to the 

 axis of the egg as a whole; the direction of telokinetic movements is such that 

 the centrosomes always move toward the animal pole and toward the point where 

 the previous cell constriction began. Consequently successive cleavage planes 

 are not at right angles to one another, but the cleavages take place in a "spiral" 

 manner. A further consideration of this subject is reserved for another paper. 



Equality or inequality of division is due in part to the localization of different 

 ooplasmic substances, and in part to local reductions in the tension of the cell 

 membrane. Indeed telokinetic movements, and consequently the direction of 

 division as well as the equality or inequality of daughter cells, are associated with 

 local variations of tension in the cell membrane. 0. Hertwig (1892) long ago 

 maintained that the spindle lies in the direction of least resistance in the cell; cer- 

 tainly this is not the principal factor which determines the position of the cleav- 

 age plane, as Jennings (1896), Zur Strassen (1896) and I (1897) have shown. 

 Doubtless the spindle lies in the axis of least resistance in many cases, but the 

 position of the spindle does not make that axis the one of least resistance. It is 

 probably owing to local reductions in the tension of the cell membrane that there 

 is an axis of least resistance, and these reductions of surface tension are not caused 

 by the position of the spindle, but vice versa. Indeed the characteristic move- 

 ments of the cell contents in all stages of division (prokinesis, metakinesis, telo- 

 kinesis) and consequently the initial direction of the subsequent spindle, as well 

 as the equality or inequality of cleavage, are probably associated with local 

 variations in the tension of the cell membrane. But more important than any 

 of these factors are the mitotic movements of the cell contents, which begin with 

 the dissolution of the nuclear membrane in the prophase and which carry the 

 spindle into certain definite positions in the cell and there bring about constric- 

 tion of the cell body in a particular plane which usually passes through the 

 equator of the spindle. 



The position of the spindle, then, is due to at least four factors viz: (1) The 

 separation of daughter centrosomes at right angles to the cell axis and in the 

 plane which separates the gonomeres. (2) Telokinetic movements by which 

 the cell axis undergoes regular changes with respect to the axis of the egg as a 

 whole. (3) Local reductions of tension of the cell membrane in certain axes, or 

 increase of tension in other axes, by which an axis of least resistance is established 



