246 BIOLOGY: GENERAL AND MEDICAL 



Upon superficial examination the theory closely resem- 

 bles the theory of Darwin, but differs from it in certain 

 important points. Thus, though Brooks agrees with 

 Darwin that gemmules are given off by all the cells of 

 the body and that they circulate in the blood from 

 which they concentrate in the germ cells, he differs in 

 ascribing to the male germ cell a strong affinity or attracting 

 power over the gemmules so that it collects a special 

 mass of them and stores them up. The egg cell is the 

 conservative principle which controls the transmission 

 of the purely racial or specific characters, whereas the 

 sperm cell is the progressive element which causes varia- 

 tion. The two kinds of germ cells are charged with 

 gemmules in different degrees. The theory is chiefly 

 aimed at the explanation of variation which is supposed 

 to be caused by every gemmule of the spermatozoon 

 uniting with that particular gemmule of the ovum that 

 is destined to give rise, in the offspring, to the cell which 

 corresponds to the one which produced the germ or 

 gemmule. When this cell becomes developed in the 

 body of the offspring it will be a hybrid and will there- 

 fore tend to vary. 



It will be observed by the thoughtful reader that with 

 the progress of knowledge more attention was being 

 devoted to the germ cells and less to the somatic cells. 

 This will be made more clear by the thoughts expressed 

 in the ensuing theories. 



Nageli conceived that the body was made up of two 

 different materials, the trophoplasm or nutrient plasm, 

 and idioplasm or germ plasm. The latter, though present 

 in small quantity, determines the detailed construction 

 of the former. He conceived the idioplasm to form a 

 very fine network of fine fibers which traverse the cells, 

 continuing from cell to cell so that all parts of the body 

 become pervaded by it as a connected network. Proto- 

 plasm, including both trophoplasm and idioplasm, he 

 considered to be compounded of exceedingly minute 

 units no larger than a molecule of albumen, to which he 

 gave the name micella. These were capable of multi- 



