The Cellular Basis 1-7 



organism, is fundamental in all modern studies of heredity. It was 

 especially emphasized by Weismann in his germ-plasm theory 



and recently it has been made prominent by Johannsen under the 

 terms "genotype" and "phenotype"; the genotype is the funda- 

 mental hereditary constitution of an organism, it is the germinal 

 type; the phenotype is the developed organism with all of its 

 visihle characters, it is the somatic type. 



But important as this distinction is between germ and soma 

 it has sometimes been overemphasized. This is one of the chief 

 faults of Weismamrs theory. The germ and the soma are generi- 

 cally 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 

 undeveloped in the germ cells. But the germ cells do not remain 

 undifferentiated; both egg and sperm are differentiated, the for- 

 mer for receiving the sperm and for the nourishment of the em- 

 bryo, the latter for locomotion and for penetration into the egg. 

 But while the differentiations of tissue cells are usually irreversi- 

 ble, so that they do not again become germinal cells, the differen- 

 tiations of the sex cells are reversible, so that these cells, after 

 their union, again become germinal cells. The ovum loses its 

 power to form yolk and during the early development it gradually 

 loses all the yolk which it had stored up ; the spermatozoon loses 

 its highly differentiated tail or locomotor apparatus and its small 

 compact nucleus absorbs substance from the cytoplasm of the 

 egg and becomes a large germinal nucleus. 



Chromatin is Germplasm, Cytoplasm is Somatoplasm. — In many 

 theories of heredity it is assumed that there is a specific "inheri- 

 tance material," distinct from the general protoplasm, the func- 

 tion of which is the "transmission" of hereditary properties from 

 generation to generation, and the chief characteristics of which 

 are independence of the general protoplasm, continuity from 

 generation to generation and extreme stability in organization. 



