Heredity. 149 



occurred, a portion of the unchanged ovum was insu- 

 lated to continue the constancy of the species. 



In this aspect the reproductive cells form a continuous 

 chain, and the reproduction of like by like is as natural 

 and necessary as it is in the Protozoa. No special theory 

 is required. Similar material in similar conditions pro- 

 duces similar results. But a serious difficulty besets 

 this doctrine. Such an early appearance and insulation 

 of the reproductive cells, continuous with the very ovum 

 itself, does indeed occur, and where it does this part of 

 the problem of heredity is simple. Early origin of special 

 germ -cells, distinguished from those of the general 

 ' ' body", has been observed in leeches, Sagitta, thread- 

 worms, many Polyzoa, Moina among crustaceans, not 

 a few insects, Phalangidae among spiders, and the Tele- 

 ostean fish Micrometrus aggregatus, while indications 

 of the same early separation are not wanting in a 

 number of other organisms. But it must be distinctly 

 allowed that in most cases it is only after differentiation 

 is relatively advanced that the future reproductive cells 

 make their appearance. Thus we have to pass from 

 the cases of the continuity of the germinal cells, to the 

 more general, but less objective fact of the "continuity 

 of the germ-plasm ". 



Weismann's Theory. Weismann, like the previous 

 investigators, reached his conclusion independently. In 

 the fact of continuity between the reproductive elements 

 of generations, the solution of likeness must be found. 

 But a direct chain of cellular continuity has been demon- 

 strated only in a few cases. The solution which is pro- 

 posed for the majority of cases is as follows : 



(1) "In each development a portion of the specific germ- 

 plasm (Keimplasma\ which the parental ovum contains, is not 

 used up in the formation of the offspring, but is reserved un- 

 changed for the formation of the germinal cells of the following 

 generation." 



(2) What is actually continuous is the germ-plasm " of definite 

 chemical and special molecular constitution ". A continuity of 

 germinal cells seems to be relatively rare; a continuity of intact 

 germ-plasms is constant. 



(3) This germ-plasm has its seat in the nucleus, is extremely 



