178 FOOT-NOTES TO EVOLUTION. 
and determine the histological structure of the cell. 
But 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- 
tecture of the chromosome, are dis- 
tributed 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, 
“micelle” with Negeli, ““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 umber, preparatory 
to the fusion of the germ cells, becomes 
of the greatest significance; to those 
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 amount 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 
‘ The ultimate 
vital units. 
Significance of 
reduction. 
