THE PHYSICAL BASIS OF HEREDITY. 



177 



Fifteen years ago Wilhelm Roux showed with cor.yi icing 

 clearness that the compHcated facts of nuclear division, 

 the careful longitudinal halving of the chromatin thread 

 and its equal distribution between the 

 Theories as to j^q daughter cells, can only be explained 

 structure an sig- ^^ ^^^ basis that the chromosomes pos- 

 nmcance of the . ,. 



chromosomes ^^^^ different structure m 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 I'danf 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 biophors, 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. 



