120 THE CHROMOSOMES 



resting period of the cells, but this must be received 

 with some caution. In many animals and in some 

 plants the chromosomes are of very different sizes 

 and shapes, and many, or even all of them, can be 

 identified at each division. It is found that these size 

 relations hold throughout all divisions of the cells. 

 While this evidence appears at first sight to show 

 that the chromosomes are structures that perpetuate 

 themselves, preserving their identity, yet it might be 

 maintained, in fact it has been maintained, that each 

 species has its own pecuhar protoplasm from which 

 chromosomes of a particular kind and number are, 

 as it were, crvstallized out anew before each cell 

 division. This point of view can not, however, be rec- 

 onciled with the evidence that follows. In Meta- 

 podius, Wilson has found that individuals may differ 

 in the particular chromosome that he calls the m 

 chromosome. While the normal individuals have a 

 pair of m chromosomes, one individual had three 

 m's; but all of the cells of any given individual 

 have the same number. These chromosomes furnish 

 strong support of the continuity of the chromosomes; 

 for, in whatever number they enter the individual 

 during fertiUzation, they retain that number through- 

 out all the subsequent generations of cells. The same 

 is true, of course, for the sex chromosomes. 



Corroborative proof is found in certain hybrids, 

 w^here the evidence is even more significant, because 

 in such cases the chromosomes introduced by the 

 male are, as it were, in a foreign medium. For 

 example, Moenkhaus first pointed out that when 



