THE CELLULAR BASIS OF HEREDITY 107 



the early cleavage stages of the egg. In other cases the germ cells are 

 first recognizable at later stages, but in practically every case they arise 

 from germinal or embryonic cells which have not differentiated into 

 somatic tissues. Germinal continuity and somatic discontinuity of suc- 

 cessive generations in sexually produced organisms is not a theory but 

 an established fact. In general, germ cells do not come from differen- 

 tiated somatic cells, but only from undifferentiated germinal cells, and 

 if in a few cases differentiated cells may reverse the process of develop- 

 ment and become embryonic cells and even germ cells it does not destroy 

 this principle of germinal continuity and somatic discontinuity. 



Thus the problem which faces the student of heredity and develop- 

 ment has been cut in two ; he no longer inquires how the body produces 

 the germ cells, for this does not happen, but merely how the latter pro- 

 duce the body and other germ cells. The germ is the undeveloped 

 organism which forms the bond between successive generations; the 

 person is the developed organism which arises from the germ under 

 the influence of environmental conditions. The person develops and 

 dies in each generation; the germ plasm is the continuous stream of 

 living substance which connects all generations. The person nourishes 

 and protects the germ, and in this sense the person is merely the carrier 

 of the germ plasm, the mortal trustee of an immortal substance. 



This contrast of the germ and the person, of the undeveloped and 

 the developed organism, is fundamental in all modern work on heredity. 

 It was especially emphasized by Weismann in his germ plasm theory 

 and recently it has been given prominence by Johannsen under the 

 terms genotype and phenotype ; the genotype is the fundamental hered- 

 itary constitution of an organism — it is the germinal type; the pheno- 

 type is the developed organism with all of its visible characters — 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 undeveloped 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 penetra- 

 tion into the egg. But while the differentiations of tissue cells are 

 usually irreversible, so that they do not again become germinal cells, the 

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

 their union, again become germinal cells. 



In many theories of heredity it is assumed that there is a specific 

 "inheritance material," distinct from the general protoplasm whose 

 function is the " transmission " of hereditary properties from generation 



