132 THE PHILOSOPHY OF BIOLOGY 



hypothesis. Notice that but for the physical concep- 

 tion of matter as made up of molecules and atoms the 

 mosaic-theory would hardly have shaped itself in the 

 minds of biologists. But this notion of matter con- 

 sisting of corpuscles must have suggested that the 

 essential ' ' living material ' ' of the organism consisted 

 also of corpuscles, as soon as a microscope powerful 

 enough to see the chromatic granules was turned on a 

 dividing cell prepared so as to render these bodies 

 visible. Obviously the primordial ovum contained all 

 the elements of the organisms into which it was going 

 to develop. But then in the process of division of the 

 ovum all these chromatic granules are shared out among 

 the cells, and a really very pretty mechanism comes 

 into existence for this purpose of distribution. 



Weismann built up his hypothesis of the germ-plasm 

 upon the observations we have outlined. The chro- 

 matic matter of the nucleus consists of elements called 

 determinants, the determinants themselves being com- 

 posed of ultimate bodies called biophors. Each de- 

 terminant possesses all the mechanism, or factors, 

 necessary for the development of a part of the body : 

 there are determinants for muscles, nerves, connective 

 tissues, for the retina of the eye, for hairs of each colour, 

 for the nails, and so on. All these determinants are 

 contained in the chromatin of the nucleus of the egg, 

 and in the divisions of the latter they are gradually 

 separated so that ultimately each cell of the larva 

 contains the determinants for one individual part, or 

 organ, or organ-system of the adult body. The right 

 blastomere, for instance, contains all the determinants 

 for the right side of the frog's body, those for the left 

 side being contained in the left half. The process of 

 cell-division involved in the segmentation of the egg 

 consists then in the orderly disintegration of this 



