THE CHROMOSOMES 



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

 fact that for any particular kind of animal of plant somes 



definite in 

 number 



FIG. 12. 



Draining by C. E. Allen 

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



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

 tions to this general statement do not mvalidale 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 pHrlciple. 

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

 of the mosquito may contain many more chromosomes tharl 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.) 



