IV. i INITIAL STRUCTURE OF THE GERM 



269 



thickness of the cell-layers is the same, the surface may be taken 

 as an index of the volume and is proportional to the original 

 volume of the fragments. Since the number of chromosomes is 

 the same, the cell-volumes should be the same, and the number 

 of cells ought to be proportional to the surface. This was, as 

 a matter of fact, found to be true in the few instances examined. 



This result is merely a restatement of Driesch's rule, that in 

 larvae developed from isolated blastomeres the surface of the 

 larvae and the number of cells are both proportional to the 

 germinal value. 



It appears, therefore, that the moment at which segmentation 

 shall end may be determined by the moment at which this 

 constant relation between nucleus and cytoplasm is reached. 



FIG. 163. , Gastrula reared from an egg of Strongylocentrotus lividus, 

 in which the sperm nucleus ha.d passed entirely into one of the first two 

 blastomeres. Optical section of a stained preparation. (After Boveri, 

 1905.) 



b, Ectoderm in the neighbourhood of the blastopore of a dispermic 

 gastrula of Strongylocentrotus lividus. (After Boveri, 1905.) 



Driesch has, however, recently reported certain exceptions to his 

 rule, amongst the mesenchyme cells in particular. These may 

 be more or less numerous than they should be, may differ in size 

 and in the size of their nuclei. It is possible, as suggested, 

 that the division of the centrosome has been suppressed (monaster) 

 with consequent reduplication of the chromosomes. Such an 

 explanation may account for the variability in the size of the 

 nuclei observed by Seeliger in his hybrids. 



In the Alga Spirogyra Gerassimow has observed a close relation 

 between the size of the nucleus and that of the cell ; here, how- 

 ever, it is the nuclear volume which is directly proportional to 

 the cell-surface. 



