62 DYNAMICS OF LIVING MATTER 



The fact that the cell division is as a rule preceded by a synthetical 

 process explains possibly the fact mentioned in the second lecture that 

 the phenomena of cell division in a ertilized egg come soon if not 

 immediately to a standstill when the atmospheric oxygen is with- 

 drawn from the egg. We have mentioned Schmiedeberg's view in 

 regard to the r6le of oxygen in synthetical processes. But even if this 

 view were not correct, we can understand that lack of oxygen might 

 indirectly interfere with the synthesis of the nuclein compounds. 



E. P. Lyon has shown that the chemical conditions and processes 

 in the cell differ in the various phases of cell division. He found that 

 during different stages of cell division the egg of the sea urchin shows 

 a different resistance to the effects of HCN, and this difference repeats 

 itself during each of the successive segmentations. More recently he 

 has added the important fact that the production of COj on the part 

 of the egg of the sea urchin also undergoes periodic variations during 

 segmentation. 



If we now turn to the physical side of the phenomena of cell division, 

 we shall meet almost the same uncertainty which confronted us in the 

 case of muscular contraction. We shall therefore confine ourselves to 

 the enumeration • of a few facts, with occasional hints for possible 

 further work. 



When we watch the process of cell division in an egg, we can dis- 

 criminate at least three distinct phases of this process : first, the ap- 

 pearance of systems of radiation — the so-called astrospheres — in the 

 protoplasm of the cell. The second phase is the disappearance (Hquefac- 

 tion?) of the nuclear wall, and the division and migration of certain 

 constituents of the nucleus, namely, the chromosomes toward the centers 

 of the astrospheres, and the formation of two new nuclei. The third 

 phase consists in the separation of the protoplasm into two pieces in 

 a plane, which, from the position of the astrospheres, as a rule, can be 

 predicted. This latter separation is the process of cell division proper. 



If newly fertilized eggs of the sea urchin are put into sea water, 

 whose osmotic pressure has been adequately raised by the addition of 

 some salt, e.g. NaCl, or sugar, no segmentation occurs as long as the 

 eggs remain in this solution. If they are brought back from this solu- 

 tion into normal sea water, they will segment, provided they have not 

 been left too many hours in the hypertonic sea water. There is, how- 

 ever, a characteristic difference between this segmentation and the 

 normal segmentation. If the eggs are brought back into normal sea 

 water after two hours, they do not divide, as a rule, first into two, and 

 then into four cells, but into three or four cells simultaneously. If they 

 are left for three or four hours in the hypertonic solution, and then 



