268 INTERNAL FACTORS IV. i 



normal (amphikaryotic) embryos or larvae of the same stage. 

 In diplokaryotic larvae, on the other hand, they are larger and 

 fewer ; while in larvae in which one portion is amphikaryotic, 

 the other hemikaryotic, the two regions are distinctly marked off 

 by the difference in the size and number of their nuclei (Fig. 163). 



The exact relation between the number of chromosomes origin- 

 ally entering into the composition of the nucleus and the 

 dimensions of the nucleus and the cell has been ascertained by 

 Boveri to be as follows : 



I. The surface-area of the nuclei is directly proportional to 

 the number of chromosomes. The chromatin, it is worth 

 noticing, is often placed beneath the nuclear membrane. 



FIG. 162. Dispermic eggs of Strongylocentrotus Hindus. A, The four 

 centres have united to form a tetraster. B, The two pairs of centres 

 have remained apart to form two independent spindles, one of which 

 contains the female and one of the male nuclei, the other the remaining 

 male nucleus. (After Boveri, 1905.) 



II. The number of nuclei is inversely proportional to the 

 number of chromosomes ; but 



III. Cell-volume is inversely proportional to the number of 

 cells in larvae of the same stage developed from whole eggs 

 of the same size ; hence 



IV. Cell-volume varies directly with the number of chromo- 

 somes and with the surface area of the nucleus. This Boveri terms 

 the fixed relation between nucleus and cytoplasm. 



V. It follows from I and IV that nuclear- volume increases 

 more rapidly thon cell-volume. 



VI. Conversely, the law holds good of egg-fragments of 

 different sizes but containing the same number of chromosomes. 



Since, in the larvae developed from such fragments, the 



