THE CHROMOSOMES 63 



3. On examining these chromosomes, we note the Thechromo- 

 fact that for any particular kind of animal or plant JJSSein 



number 



FIG. 12. 



Drawing by C. E. Allen 

 Chromosomes in cell of lily (LUium canadense), greatly magnified. 



there is a definite number in each cell. The two excep- 

 tions to this general statement do not invalidate the 

 rule, but when explained are seen to be quite in harmony 

 with it. One is, that the number may be slightly dif- 

 ferent in the two sexes ; the other, that the gametes, or 

 cells uniting in the process of fertilization, contain only 

 half the number characteristic of the species. 1 The 

 number of chromosomes in different organisms differs 

 greatly ; thus the cells of a certain parasitic worm have 



1 Other exceptions recently noted do not invalidate the general principle. 

 Miss Caroline M. Holt has found that the cells in the intestine of the pupa 

 of the mosquito may contain many more chromosomes than are normal for 

 the species, but the numbers are always multiples of three. The chromo- 

 somes have increased by division without the usual accompaniment of cell 

 division. Such cells degenerate or disintegrate, and are absorbed as nutri- 

 ment by the cells of the developing adult. (Journal of Morphology, Septem- 

 ber, 1917.) 



