THE PHYSICAL BASIS OF HEREDITY. 177 
Fifteen years ago Wilhelm Roux showed with convincing 
clearness that the complicated facts of nuclear division, 
the careful longitudinal halving of the chromatin thread 
: and its equal distribution between the 
Theories asto two daughter cells, can only be explained 
structureandsig- on the basis that the chromosomes pos- 
nificance of the ; : . 
chromosomes,  S€88 different structure in different parts 
of their extent, and that these structures, 
representing tendencies in development, are distributed 
in definite ways to the daughter cells. Were this not 
the case a simple direct mass division of nucleus and 
cytoplasm instead of the complicated process of Karyo- 
kinesis with its consequent much greater expenditure of 
energy would serve all purposes. 
The theories of Weismann are all based upon an ex- 
tension of Roux’s ideas. Briefly, he assumes a definite 
architecture of the chromatin filament, each nuclear rod 
or zdant being composed of a number of “ancestral 
germ plasms or ids, the vital units of the third order. 
Each id in the germ plasm is built up of thousands or 
hundreds of thousands of determinants, the vital units 
of the second order, which in turn are composed of 
the actual bearers of vitality or dophors, the ultimate 
vital units. The biophors are of various kinds, and 
each kind corresponds to a different part of a cell; 
they are therefore the bearers of the characters or 
qualities of cells. Various but perfectly definite num- 
bers and combinations of these form the determi- 
nants, each of which is the primary constituent of a par- 
ticular cell, or of a small or even large group of cells— 
e. g., blood corpuscles.” 
“ These determinants control the cell by breaking up 
into biophors, which migrate into the cell body through 
the pores of the nuclear membrane, multiply there, ar- 
range themselves according to the forces within them, 
