ly^ iFOOT-NOTES TO EVOLUTION. 



and determine the histological structure of the cell. 

 BiiL they only do so after a certain definitely prescribed 

 period of development, during which they reach the cell 

 which they have to control." (Weismann, The Germ 

 Plasm, pp. 75, 76.) 



Cell division, then, is a process of qualitative analysis 

 through which the determinants, in virtue of possessing 



a certain definite location in the archi- 

 The ultimate tecture of the chromosome, are dis- 

 vital units. •,,,-, , ■ r 



tnbuted ultimately to that portion of 



the body which they are to direct. Weismann has devel- 

 oped this theory to a most elaborate degree of compli- 

 cation in explaining the various phenomena of heredity 

 — to a degree, it need hardly be remarked, which passes 

 far beyond our present knowledge of the facts of cytol- 

 ogy. Just as the chemist and physicist, however, are 

 forced to the assumption of the existence of ultimate 

 atoms and molecules to explain the phenomena of non- 

 living matter, so the biologist must in some form or 

 other assume the reality of ultimate self-propagating 

 vital units, be they called " biophors " with Weismann, 

 " micella " with Nsegeli, "pangenes" with De Vries, 

 " plasomes " with Wiesner, or " physiological units " with 

 Herbert Spencer 



In the light of this probable individuality and mor- 

 phological organization of the chromosomes the method 



of their reduction in number^ preparatory 

 Significance of ^^ ^^^ {^^^xon of the germ cells, becomes 

 reduction. , , ■ -t- 1 



of the greatest significance ; to thos« 



who may deny this individuality and definite architec- 

 ture, the phenomena can have no great importance save 

 as concerns a general mass reduction in the atnount of 

 the chromatin present in the germ nuclei. It may be 

 assumed as true, in the majority of cases now accu- 

 rately known, that the reduction takes place somewhere 



