292 



THE AMERICAN NATURALIST [Vol. XLV 



demonstration by Bonnevie, 11 that while the identity of 

 the old chromosome is lost in the resting nucleus, never- 

 theless, each new chromosome arises by a kind of endo- 

 genous formation from within the substance of its im- 

 mediate predecessor, is a good point in evidence. That 

 mere number of chromosomes is not of fundamental 

 importance is evidenced by the considerable number of 

 known cases in which closely related species may be 

 characterized by a considerable difference in the number 

 of chromosomes. In my own researches on man and 

 certain birds, I have shown that instead of eight, the 

 expected number of chromosomes in spermatocytes of 

 the second order, only four (disregarding the accessory 

 which may be present) appear, but that they are ap- 

 parently bivalent in nature. 



As associated with embryonic development we should 

 have to suppose that there are considerable numbers of 

 these initial ferments, which, however, need not all be 

 present in an active condition. Certain ones required 

 for the first stages of development might well be sup- 

 posed in the course of their activities to produce or free 

 others, or activate them at the proper time to take up 

 their part in the progressive chemical activities of de- 

 velopment. It is probable, too, that many of the fer- 

 ments of the fully developed organism peculiar to the 

 special tissues have not existed as such in the germ-cell 

 at all, but have arisen at a later stage in the cells they 

 occupy as the outcome of the metabolic activities of the 

 tissue cell itself. It is a current belief, indeed, that each 

 kind of cell has its own specific ferments whereby it 

 shapes up from the common food supply submitted to it 

 in the lymph the substances necessary for its own intra- 

 molecular assimilation. Many intracellular enzymes 

 are now known to exist and it is probable that proteo- 

 lytic enzymes at least are found within the cells of all 

 living tissues. This is demonstrated by the fact of 

 autolysis, or the self-digestion of living tissues which 



Arch. ZeUforsch., Bd. I, pp. 450-514, 3908. 



