The Cellular Basis 169 



between successive generations. But in addition to these specific 

 facts there are certain general considerations 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 individual difference between organisms must be due to 

 one or more differentiating 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 experi- 

 ence 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. 



Environment Non-specific. — On the other hand different en- 

 vironmental conditions may be associated with similar develop- 

 mental results. Loeb and others have found that artificial par- 

 thenogenesis may be induced by a great variety of environmental 

 stimuli, vis., by salt solutions, by acids and alkalis, 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 produced by magnesium salts, alcohol, chlore- 

 tone, chloroform, and ether, and to this list McClendon has 

 added various other salts and anaesthetics. In all such cases it is 

 evident that the specific results of such treatment are due to a 

 specific organization of the germ rather than to specific stimuli. 



