SEC. I CONTINUITY OF GERM-PLASM 9 



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 whok' 

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

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

 experiences during life; that they can therefore be trans- 

 mitted from generation to generation unchanged. G. Jiiger ^ 

 and Nussbaum^ 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, Lehrhuch der allg. Zoologie, Leipzig, 1878, Bd. ii. 



2 M. Nussbaum, Die Differenzimng des Geschlechts im TJiierreich, Arch. f. 

 mikAnat, 1880, Bd. xviii. 



