2 32 THE POPULAR SCIENCE MONTHLY 



THE CELLULAE BASIS OF HEEEDITY AND 

 DEVELOPMENT. II 



By Professor EDWIN GRANT CONKLIN 



PRINCETON UNIVERSITY 



C. The Mechanism of Heredity 



The mechanism of heredity, as contrasted with the mechanism of 

 development, consists in the formation of particular kinds of germ cells 

 and in the nnion of certain of these cells in fertilization. We have 

 briefly traced the origin, maturation and union of male and female sex 

 cells in a number of animals, and in these phenomena we have the 

 mechanism of the hereditary continuity between successive generations. 

 But in addition to these specific facts there are certain general consider- 

 ations which need to be emphasized. 



I. The Specificity of Germ Cells 



The conclusion is inevitable that the germ cells of different species 

 and even those of different individuals are not all alike. Every in- 

 dividual difference between organisms must be clue to one or more dif- 

 ferentiating causes or factors. Specific results come only from specific 

 causes. These causes may be found in the organization of the germ 

 cells or in environmental stimuli, i. e., they may be intrinsic or extrinsic, 

 but as a matter of fact experience has shown that they are generally 

 intrinsic in the germ. In the same environment one egg becomes a 

 chicken and another a duck ; one becomes a frog, and another a fish, and 

 another a snail; one becomes a black guinea-pig and another a white 

 one ; one becomes a male and another a female ; one gives rise to a tall 

 man and another to a short man, etc. Since these differences may occur 

 in the same environment they must be due to differences in the germ 

 cells concerned. 



On the other hand, different environmental conditions may he asso- 

 ciated with similar developmental results. Loeb and others have found 

 that artificial parthenogenesis may be induced by a great variety of 

 environmental stimuli, viz., by salt solutions, by acids and alkalies, by 

 fatty acids and fat solvents, by alkaloids and cyanides, by blood serum 

 and sperm extract, by heat and cold, by agitation and electric current. 

 There is certainly nothing specific in these different stimuli. Similarly, 

 Stockard has discovered that cyclopia, or one-eyed monsters, may be pro- 

 duced by magnesium salts, alcohol, chloretone, chloroform, and ether. 

 In all such cases it is evident that the specific results of such treatment 



