HISTORICAL SKETCH 443 



of the male parent were brought in by the nucleus of the spermatozoon. 

 During the long series of nuclear divisions beginning with that in the 

 zygote and ending with the completion of the mature organism, the 

 many kinds of determinants are sorted out through a progressive dis- 

 integration 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 on the proto- 

 plasm, 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. 



Weismann accounted for the presence of a complete outfit of deter- 

 minants in the gametes and zygote, and hence for the phenomenon of 

 hereditary resemblance, by assuming that a certain portion of the 

 complete germ-plasm is carried along unchanged during ontogenesis and 

 is delivered intact to the germ-cells. He thus rejected Darwin's sugges- 

 tion that representative particles (gemmules) were transmitted from all 

 the body-cells to the germ-cells. It had been shown that in certain 

 animals the primitive germ-cells are set aside at once when development 

 begins, and Weismann pointed out that they are therefore differentiated 

 before any sorting out of the hereditary units could have taken place. 

 Hence the germ-cells are really produced by the germ-cells of the previous 

 generation and not by the individual's own soma (body) ; they are present 

 from the beginning of development with the full hereditary outfit, and 

 by a few equational divisions give rise to the gametocytes. In the case 

 of those animals and plants whose germ-cells appear later in the ontogeny, 

 Weismann held that, although a sorting out of the units occurs in the 

 majority of the cells during ontogenesis, those meristematic cells which 

 constitute the chain connecting the zygote with the germ-cells — the 

 germ-track (iiLem6a/in)— maintain the undiminished germ-plasm. Hence 

 in all cases there is a continuity of the germ-plasm, if not a continuity of 

 the germ-cells (unless meristematic cells also are germ-cells). 



Weismann argued that since there is no contribution of hereditary 

 elements from the soma to the germ-cells, somatic changes being in no 

 way impressed upon the germ-cells from which the next generation is to 

 arise, there can be no inheritance of acquired somatic modifications. 

 In multicellular animals the only inherited variations are those origi- 

 nating in the germ-plasm of the germ-cells or germ-track as responses to 

 internal (nutritive, etc.) or external environmental stimuli and those due 

 to recombinations of hereditary units at the time of syngamy (amphi- 

 mixis). He admitted that the germ-plasm, though remarkably stable, 

 might be altered directly by the environment (parallel modification of 



