64 DYNAMICS OF LIVING MATTER 



cell division. Under normal conditions, the growth of the chromo 

 somes is followed by a formation of astrospheres and division of the 

 nuclei, and this in turn is followed by a cell division. The loss of water 

 which the egg undergoes in the hypertonic sea water seems to inter- 

 fere mostly with the cell division. It is possible that the viscosity of 

 the protoplasm is increased by the loss of water, and that this condi- 

 tion interferes somewhat with the migration of chromosomes after they 

 have divided and still more with the segmentation of the protoplasm. 

 The growth of the chromosomes and the subsequent formation of as- 

 trospheres seem, however, to continue for some time in the hypertonic 

 sea water. 



O. and R. Hertwig (as well as Roux) have noticed that as a rule 

 the plane of division of a non-spherical cell is at right angles with the 

 direction of the greatest diameter, or extension of the cell. Driesch 

 has given a nice experimental proof for this rule. If the newly fer- 

 tilized egg of the sea urchin be gently pressed under a cover glass, so 

 that it is slightly flattened, the plane of division is at right angles to the 

 slide. The position of the plane of cleavage is determined by the 

 position of the nuclear spindle, and the latter depends upon the position 

 of the centrosomes or astrospheres. The question hence arises, How 

 does it happen that in most cases the common diameter of the two 

 astrospheres coincides with the longest diameter of a cell? This posi- 

 tion of the astrospheres or centrosomes becomes comprehensible on the 

 assumption that these organs not only repel each other, but are also 

 repelled by the external surface of the nuclei and the inner surface of 

 the cell limit. The forces involved in this repulsion must be forces 

 such as occur in liquids, as the contents of the egg of the sea urchin 

 is mainly liquid. It must, moreover, be taken into consideration that 

 the space in which the process of cell division occurs, is generally of 

 microscopic and always of capillary dimensions. It is therefore quite 

 possible that the repelling forces in this case are capillary forces. There 

 is, however, another fact to be considered ; namely, that in the process 

 of cell division the egg of some animals becomes elliptic, with its long 

 axis falling in the direction of the common diameter of both astro- 

 spheres. This has given rise to the idea that the spindle or the astro- 

 spheres elongated the egg. I have often noticed - - as others have 

 undoubtedly done before me an elongation of the egg of the sea 

 urchin in the direction of the spindle, but this always occurred imme- 

 diately before the cell division. It gives easily the impression as if 

 contractile forces were active in radial directions in the astrospheres 

 and that these forces had something to do with the process of cell divi- 

 sion. Certain deviations from Hertwig's law may be only apparent. 



