II CLEAVAGE 55 



It is clear that the surf ace- area of the nucleus is directly 

 proportional to the number of chromosomes contained in it. 



It follows therefore at once that the size of the nucleus (as 

 measured by its surface-area) varies inversely with the number 

 of cells. But in equal areas of like tissues (which are of the 

 same thickness) the number of cells must be inversely propor- 

 tional to the size (volume) of the cells. Hence, the cell-volume 

 is directly proportional to the surface-area of the nucleus, as 

 well as to the number of chromosomes contained in it, or the 

 ratio of plasma to nucleus has in the blastula stage, and again 

 in each tissue at later stages, some constant value. 



It is suggested that the attainment of this constant value as 

 a result of the multiplication of the nuclei during segmenta- 

 tion brings this process as such to an end. 



The converse of this experiment is seen if the number of 

 cells is compared in larvae reared from fertilized nucleate egg- 

 fragments of different sizes. 



Two cases may be quoted. 



Ratios of surfaces Ratio of numbers of nuclei 

 of gastrulae. in gastrulae. 



1. 1:6-5 1:148 



2. 1:1.5:2-8 1:142:2-82 



The number of nuclei being proportional to layers of equal 

 thickness the cell-volume has, at a given stage, a constant 

 value. This is, of course, a re-statement of Driesch's rule that 

 in larvae developed from isolated blastomeres the number of 

 cells and the surface-area are both directly proportional to the 

 germinal value. 



It will be observed that the value of the constant plasma- 

 nucleus ratio has not been given by Boveri, and further that 

 the actual volume of the cells has not been determined. This 

 lacuna in our knowledge has been filled by the researches of 

 Fraulein Erdmann, who has investigated in the same form 

 as that employed by Boveri (Strongylocentrotus lividus) the 

 changes in dimensions undergone by nucleus, cell, and chromo- 

 somes, during early development. 



In the accompanying table (Table VII) the results for one 

 temperature 10 C. are given, for a series of stages. 



