DEVELOPMENT 209 



arrests one's attention. And, accepting this general result, we 

 find that the exceptions to it can be explained by logical 

 hypotheses. 



In Weismann's time it was shown that the nucleus had a certain 

 architecture and this was amplified — far beyond the observational 

 bases. It was seen, occasionally, that each chromosome could 

 be decomposed into discrete granules and these, or more minute 

 parts that were not actually seen, were called ids — each id being 

 regarded as involving all the characters of a complete organism. 

 The ids were regarded as being made up of (ultramicroscopic) 

 parts — the Determinants, each Determinant involving all the 

 characters of an organ, tissue, part, or character. The Determin- 

 ants were supposed to be made up of Biophores which were 

 regarded as the ultimate living, material particles, corresponding, 

 in a kind of way, to molecules. 



The materials of which the cell-chromatins were composed, 

 were the germ-plasm. This, if not an absolutely stable substance, 

 was very nearly so and hypotheses were devised to account for 

 changes in the natures of the germ-plasmatic elements, or 

 determinants. But novelties in the history of a race of organisms 

 — or what is called transformis?n — were regarded as due, not so 

 much to changes in the determinants as to reassortments of the 

 latter occurring in every act of sexual conjugation. In itself, as 

 a complex system of chemical substances the germ- plasm was so 

 nearly stable that it was unaffected by the environmental changes 

 to which the soma or body was exposed. But whatever qualities 

 the germ-plasm had were due to its chemical constitution. 



Weismannism included a logical hypothesis of development. 



The chromosomes were regarded as linear arrangements of 

 determinants and in an ordinary cell-division it was seen that 

 the nucleus divided in such a way that all its visible parts were 

 halved between the two daughter-cells. Thus let us suppose 

 that a nucleus contains the determinants, a, b, c, d, e,f : when it is 



about to divide each determinant is halved thus , 7 -l. 



a, 0, Cy ciy e,j 



One set of all the halves then goes into one daughter-cell and the 



remaining set into the other daughter-cell. Clearly, then, the 



daughter- cells are similar to the mother cell. Now apart from 



the pecuUar meiotic nuclear divisions in the maturation of the 



gonidial, or germ cells, all the cell- divisions in the course of the 



