WEISMANNISM AND OTHER THEORIES 401 



in a given species differ only slightly among themselves, the differences 

 corresponding to the variations observed within the species : they are the 

 "ancestral germ-plasms" which have been contributed by past genera- 

 tions. The ids are identified with the visible chromatin granules in the 

 nuclear reticulum or in the chromatin thread during mitosis. In most 

 cases the ids are grouped to form idants, or chromosomes. In some forms 

 which have a large number of granular chromosomes it is possible that 

 each is composed of but one id. The id therefore, rather than the 

 chromosome, is the unit of primary importance. In case there are 

 several ids in a chromosome (idant) they are arranged in a linear series. 

 The idea that the chromosomes are all alike since they carry closely simi- 

 lar ids was later (1913) modified by Weismann, largely as the result of 

 the demonstration that very minute characters are segregated in 

 Mendelian fashion. 



With the aid of this elaborate mechanism Weismann explained onto- 

 genetic development in the following manner. In the fertilized egg from 

 which the individual is to develop all the kinds of determinants are 

 present: thoes of the female parent are contained in the egg nucleus 

 and those of the male parent are brought in by the nucleus of the 

 spermatozoon. During the long series of cell-divisions beginning with 

 the fertilized egg and ending with the completion of the mature organism, 

 the many kinds of determinants are sorted out through a progressive 

 disintegration of the ids, and are distributed in a definite and orderly 

 manner to the different parts of the body. Many somatic mitoses are 

 therefore regarded not as equational (erbgleich) , but in reality qualitative 

 (erbungleich) . When a given determinant finally reaches the proper cell, 

 i.e., when that cell is finally formed, the determinant splits up into its 

 constituent biophores; and these, through their action upon the cell 

 elements, give to the cell its specific characters. The general character 

 of a cell is accordingly due to the type or types of determinant which 

 it receives. For Weismann, therefore, development (ontogenesis) was 

 definitely bound up with the evolution or unfolding of a complex struc- 

 ture contained in the fertilized egg. Although he did not hold that 

 the units in the egg have the same spatial relations as their corresponding 

 characters or structures in the adult, it has been said with some degree 

 of truth that he transferred preformationism to the nucleus. 



Such being Weismann's conception of development, how did he account 

 for heredity? If the various kinds of body cells in an individual are 

 characterized by different types of determinants, how is it that the germ 

 cells, or gametes and fertilized egg through which this individual is 

 to give rise to the next generation, possess a complete outfit of deter- 

 minants? According to Darwin's hypothesis, outlined in the foregoing 

 pages, representative particles or gemmules are contributed by all the 

 body cells at all stages to the germ cells, by which they aro transmitted 



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