HEREDITY 



421 



tions can develop into such an organism. For this idea we have strong 

 experimental evidence. It has long been known that the eggs of certain 

 species of animals can develop without fertilization, i. c, without 

 having united with a sperm or male sex-cell. In such cases there can 

 be no question that the potentialities of an entire organism are con- 

 tained in the egg, for without any outside help the egg develops into a 

 complete individual of the species. In recent years it has been shown 

 that the eggs of many species in which fertilization normally occurs 

 may by artificial means be made to develop without having united with 

 a sperm. This is true of the eggs of sea-urchins, star-fishes, and of 

 certain worms and mollusks. Such eggs artificially stimulated to 

 development produce entire individuals, similar to those produced by 

 fertilization, but possibly less vigorous. 



On the other hand, a sperm cell may be made to develop, if it is 

 allowed to penetrate into a fragment of an egg, even a fragment which 

 lacks the important cell-nucleus. In such cases the entire nuclear 

 material of the embryo is furnished by the sperm, yet the embryo so 

 produced is complete, lacking no essential part, and similar except in 

 size and vigor to normal embryos produced by fertilization. 



• Accordingly the evidence is fairly complete that each germ-cell 

 (egg or sperm), considered as the vehicle of heredity, represents a 

 complete organism, and that an individual produced by the union of 

 two such germ-cells represents twice over each heritable trait of the 

 species. In other words, the germ-cell is single, the individual is 

 double. 



This fundamental principle of the singleness of the germ in con- 



Fig. 3. An Albino male Guinea-pig, with which was mated the albino 



shown in Fig. 2. 



