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INTRODUCTION TO CYTOLOGY 



to the next generation. (See Fig. 157, A.) Such a contribution of 

 elements from the body cells to the germ cells was denied completely by 

 Weismann. He held rather that a certain portion of the complete germ- 

 plasm (idioplasm; chromatin) of the fertilized egg is carried along un- 



A 



B 



C 



FIG. 157. Diagram illustrating the hypotheses of Darwin and Weismann. The large 

 circles represent successive generations of individuals, and the small circles their germ 

 cells. For the sake of simplicity inheritance is shown as uniparental rather than biparental. 

 A, Darwin's Hypothesis of Pangenesis. The branching solid lines ending in arrows repre- 

 sent the sorting out of the hereditary units (gemmules) during ontogenesis; the dotted 

 arrows show the migration of gemmules from the body cells to the germ cells, by which 

 they are carried into the next generation. B, C, Weismann's theory of the continuity of 

 the germ-plasm, with no contribution of hereditary units from the body cells to the germ 

 cells. In one case (B) the germ cells are set aside at the beginning of ontogenesis, and in 

 the other (C) much later. In both cases the "complete germ-plasm" is delivered to the 

 germ cells through a shorter or longer series of equational divisions (heavy lines). 



changed and delivered intact to the germ cells. It had been shown 

 (Haeckel 1874; Rauber 1879; Jaeger 1878; Nussbaum 1880; Galton) 

 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 can have 



