CELL SIZE AND NUCLEAR SIZE 13 
Boveri (’02, ’05) found that the size of the nuclei in sea urchin 
larvae is dependent upon the number of chromosomes which enter 
into the nuclei; in parthenogenetic or hemikaryotic eggs the nu- 
clei are smaller than in fertilized (amphikaryotic) ones, and they 
are smaller in the latter than in diplokaryotic eggs in which the 
number of chromosomes is greater than normal. Furthermore 
he found that nuclei with a small number of chromosomes are 
not only smaller than those containing a larger number but that 
the cells in which they lie are also smaller, owing to the occurrence 
of a larger number of cell divisions in cells with small nuclei than 
in cells with large ones. 
Boveri’s work was based primarily on his studies of echinoderm 
development and some of his conclusions are not applicable, 
without modification, to the eggs and larvae of other forms, 
especially forms in which there are great inequalities of cleavage 
and in which various cells of the larva differ markedly from one 
another in size. Thus his generalization, sometimes mentioned 
as ‘Boveri’s Law,’ viz., ‘‘Die Grésse der Larvenzellen ist eine 
Funktion der in ihnen enthaltenen Chromatinmenge, und zwar 
ist das Zellvolumen der Chromosomenzahl direkt proportional,” 
could not apply, without modification, to eggs or larvae in which 
various cells differ greatly in size without any corresponding 
difference in the number of chromosomes. Unequal cell divisions 
are frequently found in the development of mollusks, annelids 
and ascidians, where purely mechanical causes, such as mutual 
pressure between cells or the pressure of yolk within cells are not 
involved; in such eases the sizes of the nuclei invariably become 
proportional to that of the plasma, though the number of chro- 
mosomes remains the same in every nucleus. Similarly, many 
cells at first equal in size become unequal through dissimilar 
growth, and their nuclei then become unequal also. Finally, 
in each of the animal groups named, cells at first equal in size may 
become unequal through dissimilar rates of division. In all 
such cases the number of chromosomes appears to be, and pre- 
sumably is, the same in every nucleus of a given egg or embryo. 
Evidently in cases of normal development the number of chromo- 
somes does not determine the varying sizes of cells and nuclei. 
