102 HEREDITY AND ENVIRONMENT 



Johannsen under the terms "genotype" and 

 "phenotype" ; the genotype is the fundamental 

 hereditary constitution of an organism, it is 

 the germinal type; the phenotype is the de- 

 veloped organism with all of its visible char- 

 acters, it is the somatic type. 



But important as this distinction is between 

 germ and soma it has sometimes been over- 

 emphasized. This is one of the chief faults of 

 Weismann's theory. The germ and the soma 

 are generically alike, but specifically different. 

 Both germ cells and somatic cells have come 

 from the same oosperm, but have differentiated 

 in different ways; the tissue cells have lost 

 certain things which the germ cells retain and 

 have developed other things which remain un- 

 developed in the germ cells. But the germ 

 cells do not remain undifferentiated; both egg 

 and sperm are differentiated, the former for 

 receiving the sperm and for the nourishment 

 of the embryo, the latter for locomotion and 

 for penetration into the egg. But while the 

 differentiations of tissue cells are usually irre- 

 versible, so that they do not again become 

 germinal cells, the differentiations of the sex 



