CONTINUITY OF GERM-PLASM 



one or more generations, e.g. the parents, being passed over 

 (atavism). 



Weismann's attempted explanation rests on the assump- 

 tion that the germ cells, by means of which inheritance must 

 be transmitted, pass over unchanged from the body of the 

 ancestor into that of the descendant ; that they form a whole 

 which so far contrasts strongly with the rest of the body ; 

 that it takes no kind of part in the changes which the latter 

 experiences during life; that they can therefore be trans- 

 mitted from generation to generation unchanged. G. Jager l 

 and Nussbaum 2 had previously already sought to explain such 

 an immediate connection between the germ-cells of parent 

 and child, by the assumption that the germ-cells of the off- 

 spring separated themselves at the very beginning of embry- 

 onic development, or at all events before any histological 

 differentiation, from the rest of the developing ovum. 



According to Weismann the assumption of Jager and 

 Nussbaum comes ultimately to the same thing inheritance 

 would be due to the fact that in every reproduction a part of 

 the germ-plasm of the parental egg-cell is not used up in the 

 construction of the offspring, but remains unchanged to serve 

 for the formation of the germ-cells of the following generation. 

 The inheriting substance lies, according to Weismann, in the 

 contents of the nucleus of the germ-cell. 



" The germ-cells," says Weismann, " arise in their essential 

 and distinctive substance, not by any means from the body 

 of the individual, but directly from the parental germ-cells." 

 " Inheritance," he continues, " takes place wholly and solely 

 because a substance of definite chemical, and above all, 

 molecular composition passes over from the germ -cells of 

 one generation to those of the next. This substance, the 



1 G. Jager, Lehrbuch der allg. Zoologie, Leipzig, 1878, Bd. ii. 



2 M. Nussbaum, Die Differenzirung des GescMechts im Thierreich, Arch. f. 

 mik Anat., 1880, Bd. xviii. 



