The Cellular Basis 125 



3. Germinal Continuity and Somatic Discontinuity. — Many 

 ingenious hypotheses Wave been devised to explain things which are 



not real, and this is one of them. The doctrine that adult organ 

 isms manufacture germ cells and transmit their characters to them 

 is now known to he erroneous. Neither germ cells nor any other 

 kind of cells are formed by the body as a whole, hut every cell in 

 the body comes from a preceding cell by a process of division, 

 and germ cells are formed, not by contributions from all parts of 

 the body, but by division of preceding cells which are derived 

 ultimately from the fertilized egg (Fig. 41). The hen does not 

 produce the egg, but the egg produces the hen and also other 

 eggs. Individual traits are not transmitted from the hen to the 

 egg, but they develop out of germinal factors which are carried 

 along from cell to cell, and from generation to generation. 



Germ Cells and Body Cells. — There is a continuity of germinal 

 substance, and usually of germinal cells, from one generation to 

 the next. In some animals the germ cells are set apart at a very 

 early stage of development, sometimes in 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. In general then germ cells do not come from differen- 

 tiated body cells, but only from undifferentiated germinal cells, 

 and if in a few doubtful cases differentiated cells may reverse the 

 process of development and become embryonic cells and even 

 germ cells it does not destroy this general principle of germinal 

 continuity and somatic discontinuity of successive generations. 



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

 velopment 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 produce the body and other germ cells. The germ 

 is the undeveloped organism which forms the bond between suc- 

 cessive generations ; the body is the developed organism which 

 arises from the germ under the influence of environmental con- 



