THEORIES OF HEREDITY 57 



(d) Weism Ann's Germ-Plasm. 



We now come to the last of the theories of heredity men- 

 tioned above — namely, the one propounded by Weismann. 

 The fundamental distinction between that part of the 

 germ which builds up the body and that which remains as 

 the germ-cells to constitute the next generation is retained 

 and further worked out. But Weismann also tried to 

 solve the problem of the orderly arrangement of the units, 

 which are the bearers of the hereditary qualities, by a 

 profound, though rather complex, hypothesis. 



Weismann elaborated his system only gradually in a 

 series of essays (published together under the title Essays 

 on Heredity and Kindred Subjects), but gave a complete 

 statement of his final views in his book. The Germ-Plasm, 

 in 1893, and once more in abbreviated form in his 

 Evolution Theory in 1904. 



Weismann distinguishes, with Naegeli, two kinds of 

 substances in the body — the " morphoplasm " (what 

 Naegeli had called the " trophoplasm "), which is the 

 living protoplasm of the cell-body, and the "idioplasm," 

 which is the bearer of the hereditary qualities of the 

 organism and the active formative element of it. But 

 while the latter, according to Naegeli, was distributed 

 throughout the cell-bodies of the organism, Weismann, 

 with the newer knowledge, relegated the idioplasm to the 

 nuclear structure of the cells. 



Weismann's units are the " Determinants " — thus called 

 because they determine the nature and quality of the cells 

 and parts of the body which they represent. They have, 

 like all living units, the power of growing and multiplying 

 by simple division. There are as many determinants as 

 there are independently variable parts of the body, be they 

 single cells, groups of cells, or even portions of cells, so long 

 as any such part is capable of varying independently from 

 any other. For as each part of the organism has its own 

 character, on the strength of the determinant which im- 



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