THE CELL AND CELL DIVISION 73 



were new and independent formations in each mitosis, they would show 

 a normal variability in number throughout long series of mitoses. 

 However, numerical variability is so rare as to be practically absent, 

 although the few exceptions known are emphasized by those who do 

 not accept the idea of continuity. Constancy of form seems to be less 

 precise than constancy of volume, but both are sufficiently marked to 

 be noteworthy. It is difficult to get precise observations here on account 

 of the liability to shrinkage or deformation of the chromosomes in the 

 preparation of the material for study. There is often undoubtedly a 

 definite pairing of chromosomes in the somatic nuclei (Fig. 33), while in 



the germ nuclei, with - chromosomes, the same categories of chromo- 







somal form and size found in somatic nuclei are distinguishable, but 

 these are no longer paired there are 

 only single representatives of each. 

 This is one of the strongest points 

 favoring this hypothesis. And no less 

 significant is the fact that the odd or 

 unpaired chromosomes associated with 

 sex differentiation, mentioned above, 

 remain constant in size and form and 

 number, throughout the tissue cells of 

 certain individuals, and are easily rec- 

 ognizable, not only through their pe- 

 culiar morphology but on account of 

 their peculiar behavior as well. It is 

 important in this connection, to no- 

 tice that these particular chromosomes 

 may remain undissolved even in the early prophase of division of a sper- 

 resting nucleus, where they had often matogonium of Brachystola magna. 

 v i MI i , 11. From Button. Spiremes forming in 



been described as chromatm nucleoh lobes of the nuc i eu s corresponding 



or other bodies, before their signifi- with the chromosomes which en- 

 cance was appreciated or their history tered the nucleus at the close of the 



J preceding division, 

 known. 



Another group of facts of quite a different character has an important 

 bearing upon these hypotheses. It sometimes happens in mitosis that 

 one or more chromosomes belonging to one daughter group, accidentally 

 become included with the other group so that one of the daughter 

 nuclei has fewer, the other more, than the normal somatic number. 

 In subsequent divisions of these cells the number of chromosomes 

 appearing is not the normal, but the increased or diminished number, 

 the sum of the two, however, always being 2s. Or in fertilization of the 



egg by the sperm, each of which has ~- chromosomes, various abnor- 



