6o DYNAMICS OF UVING MATTER 



masses of protoplasm remained in one of the two daughter cells ; some- 

 times all the chromosomes were united into one nucleus and sometimes 

 he obtained two nuclei. He found that cells with an increased mass of 

 chromatin only began to divide after their protoplasm had reached 

 a much greater mass than that found in the normal cells with half 

 the mass of the chromosomes. This fact seems to indicate that cell 

 division is determined by the ratio of the mass of the chromosomes 

 to that of the protoplasm. If the mass of the chromosomes in a cell 

 is increased, the tendency for cell division does not develop until the 

 mass of protoplasm is increased also. It is the merit of Boveri to have 

 found the law which governs this condition for cell division.* He 

 compared the process of segmentation in normally fertilized fragments 

 of sea-urchin eggs, which contained the normal number of chromo- 

 somes, with that of enucleated fragments which contained only the 

 sperm nucleus, and whose mass of chromatin was only one half of 

 that of the normal fertilized egg. In order to understand his results, 

 the reader's attention should be called to the following fact : in the 

 process of cell division each daughter nucleus of the egg contains just 

 as many chromosomes as the mother nucleus, but the mass of chromatin 

 (and possibly the other constituents of the nucleus) of each chromosome 

 in the daughter nucleus is only one half of that of the corresponding 

 chromosome of the mother nucleus. The next phase — the resting stage 

 between two divisions — consists in the growth of the chromosomes 

 of the two daughter nuclei until they have reached the mass of 

 the original chromosomes and then a new nuclear and cell division 

 begins. The material for the growth of the chromosomes is furnished 

 by the protoplasm and according to the above-quoted idea of Sachs 

 by the reserve material included in the protoplasm and not by the 

 "living" part of the latter itself. It is, however, questionable whether 

 this latter discrimination has any real basis. In this way the process 

 of cell division in the egg consists in the gradual transformation of proto- 

 plasmic into chromatin material of the nucleus until a definite ratio 

 between the mass of the chromosomes and the protoplasm is reached. 

 When this is estabhshed, no new cell divisions are possible until the 

 mass of the protoplasm is increased again through the absorption of 

 food stuffs on the part of the cell. The process of the transformation 

 of protoplasm into chromatin is necessarily rendered discontinuous 

 through the fact that the chromosomes cannot grow indefinitely, but 

 that growth will stop as soon as they have reached a Certain size, 

 and this fact leads apparently to the process of nuclear divisions. I 



* Boveri, ZelUn-Studien, Heft 5. Ueber die Abhangigkeit der Kerngrosse und Zellen- 

 zahl der Seeigel-Larven von der Chromosomenzahl der Ausgangszellen, Jena, 1905. 



