342 THE WORLD OF LIFE CHAP 



In Professor J. Arthur Thomson's most valuable and 

 illuminating work on Heredity, in which he impartially 

 expounds the theories and discoveries of all the great 

 physiological writers of the world, he gives a very high, 

 if not the highest, place to those of Weismann. I will 

 therefore quote from his volume Weismann's latest short 

 statement of his hypothesis as to the nature of the germ- 

 plasm ; and also Professor Thomson's very short summary 

 of it, giving an explanation of Weismann's special ter- 

 minology. Weismann's statement is as follows : 



" The germ-substance owes its marvellous power of development, 

 not only to its chemico-physical constitution, but to the fact that it 

 consists of many and different kinds of primary constituents, that is, 

 of groups of vital units equipped with the forces of life, and capable 

 of interposing actively and in a specific manner, but also capable 

 of remaining latent in a passive state until they are affected by a 

 liberating stimulus, and on this account able to interpose success- 

 fully in development. The germ-cell cannot be merely a simple 

 organism ; it must be a fabric made up of many different organisms 

 or units a microcosm." l 



And Professor J. A. Thomson's Summary of Weis- 

 mann's mechanics of the germ-plasm is as follows : 



SUMMARY 



" The physical basis of inheritance the germ-plasm is in the 

 chromatin of the nucleus of the germ-cell. 



" The chromatin takes the form of a definite number of chromo- 

 somes or idants (Fig. no, B, C, D, id}. 



"The chromosomes consist of ids, each of which contains a 

 complete inheritance. 



" Each id consists of numerous primary constituents or deter- 

 minants. 



"A determinant is usually a group of biophors, the minutest 

 vital units. 



" The biophor is an integrate of numerous chemical molecules." 



In the preceding Summary I have italicised the technical 

 terms invented by Weismann for the different stages of what 



development of cells up to the first cell-division. The small letters (jd) are not 

 referred to in Weismann's explanation on the plate itself, nor in his description 

 of what happens. But these letters evidently mean " idants," as explained in 

 Professor J. A. Thomson's summary of Weismann's theory at p. 20. 



1 The Evolution Theory, trans, by J. A. Thomson, 1904, vol. i. p. 402. 



