THE CELL AND CELL DIVISION 67 



or tissue cells of certain individuals; thus in the cleavage cells of 

 Ascaris megalocephala the number is two or four, in the tissues of 

 Helix pomatia, twenty-four or forty-eight, in Strongylocentrotus, 

 eighteen or thirty-six. In such cases each of the lesser number 

 is said to be bivalent, and it is supposed, not without reason, 

 that each is actually composed of two ordinary or univalent 

 chromosomes. In a few instances the number may be even 

 less than one-half the normal and each is then said to be pluri- 

 valent; thus in the formation of the embryo-sac in the lily a 

 variation in certain nuclei has been found, the number varying 

 by fours from twelve to twenty-four (s = 24). The significance 

 of these unusual cases is varied and sometimes doubtful. 

 Again, constant differences in chromosome number, in both 

 somatic and germ cells, are associated with sex differences in a 

 large number of species of several widely divergent phyla. In 

 such cases the females have a somatic group from one to five, 

 or even more, in excess of that of the male; in such cases the 

 specific number is fixed, though some variable species are 

 known. 



None of these unusual deviations from the normal is of the 

 character of a " normal variability." Indeed very few in- 

 stances of this kind of variability in chromosome number are 

 known. One instance is that of the salamander larva, in 

 certain tissue cells of which the number is said to vary (Delia 

 Valle) in different individuals from nineteen to forty, the normal 

 being twenty-four, and in a single specimen limits of nineteen 

 and twenty-seven have been described. 



This very high degree of specific numerical constancy of the 

 chromosomes indicates strongly that the appearance of a 

 chromosome in mitosis is not determined by a large number of 

 causes, but that it is the result of the operation of a single and 

 simple factor; what this may be is a matter of conjecture only. 



Another important characteristic of the specific chromosome 

 group is the constancy of the form and size differentiations 

 among the members of the group. In many organisms it can 

 be seen clearly that the chromosomes of a single nucleus are 

 not all of the same size and form; they may differ in shape, 



