blackman: the spermatogenesis of scoloi'Enkua. 55 



points upon the membrane at which these rays are inserted to come 

 together. As the traction is exerted from opposite poles of the cell, 

 the forces are so equalized that the place of this concentration of the cell 

 membrane naturally falls in the equatorial region. Thus it comes about 

 that the stress upon the cell membrane at this place is much greater 

 than at any other. The attachment of the fibres being thus limited to a 

 verv small area, it is evident that the traction thus exerted would cause 

 an annular constriction of the cell membrane at this plane. Were the 

 force exerted from only one point, the contraction of the connecting rays 

 would cause the membrane to be drawn toward that point. However, 

 this is not the case. The power proceeds from two sources, which are 

 at equal distances from the point of application and are attached at equal 

 angles to the membrane. As the fibres proceeding from one of these 

 sources form an obtuse angle with those proceeding from the other, the 

 effect of contraction is to cause the two fibres to extend in a straight line 

 between the two points. As this proceeds, the cell membrane, attached 

 firmly to these rays, is drawn inward, so that, at the time when the two 

 groups of fibres become parallel to each other, the cell membrane has 

 constricted until there is but a very small opening connecting the two 

 lobes. 



We have seen that during the constriction of the cell there has ap- 

 peared a depression in the membrane at the point where the centrosome 

 rests upon its surface. This phenomenon I believe can be explained as 

 follows: During the lengthening of the cell it appeal's that the required 

 force probably is produced by a repulsion exerted by the centrosomes 

 upon each other, causing them to separate. This explanation is no 

 longer sufficient for the subsequent changes, as the centrosomes no longer 

 continue to move apart. Therefore the later changes must be accom- 

 plished by the contraction of the archoplasm fibres. That this is true 

 is shown by two facts : the observed thickening of the fibres, and the 

 depression in the cell membrane at the place of attachment (polar 

 region). 



While the constriction of the cell wall is taking place, the interzonal 

 filaments extending between the two daughter masses of chromatin ap- 

 parently take no active part, although they may well play a passive one. 

 During the later stages the ends nearest the poles seem to lie free in the 

 cytoplasm and to have no connection whatever with the centrosomes. 

 It is evident that even at this early stage of the telophase, shortly after 

 constriction has begun, these structures are beginning to disintegrate, 

 for the distal ends — i. e. those nearest the poles — have become granular 



