58 CLEAVAGE II 



The egg used was that of Crepidula, a Mollusc, which segments 

 spirally. As segmentation progresses the plasma increases at 

 the expense of the yolk. 



The table gives the value of the nucleo-plasma ratio 



/nuclear volume \ , , 



( -, ) . as calculated from measurements taken 



> plasma volume ' 



when the nuclei first become spherical after division. 



TABLE X. 

 Cell. Ratio. 



Before cleavage 1 : 27-5 



AB or CD 1 : 13-5 



A, B, C or D 1 : 14-5 



1,4 -ID 1:12-7 



la-Id 1:14-5 



2^-2D 1:12-7 



2a-2d 1:25-6 



l^-ld 1 1:35-7 



la 2 - Id 2 1:14-6 



3^-3Z) 1:1-1 



Ba-Bd 1:14-5 



2a l -2d 1 1:10-3 



2a*-2d? 1:10-3 



la 1 - 1 -!** 1 - 1 1:14-6 



I a 1 '* -Id 1 -* 1:7-0 



There are great differences, it will be seen, in the value of 

 the ratio in different parts of the egg. The nuclear size is 

 apparently determined partly by the length of the resting 

 period the value of the ratio increases where this is pro- 

 longed, as in 3 A 3 D partly by the amount of plasma 

 present, and partly by the number of chromosomes. The 

 third factor is of course constant in all the instances quoted 

 in the table, but Conklin has shown that the chromosomes 

 may be scattered by the use of hypertonic solutions, and that 

 the nuclei so formed from less are smaller than those formed 

 from more chromosomes. The influence of the second factor 

 can be demonstrated by experiment, for if the egg be centri- 

 fuged the yolk may be unequally distributed between the 

 first two blastomeres. The blastomere with the larger share 

 of cytoplasm has also a larger nucleus and a larger 

 centrosome. 



The mean value for the nucleo-plasma ratio of all the 

 blastomeres is 1:15. Measurements made on adult tissue 



