THE CELLULAR BASIS 99 



comes from a preceding cell by a process of di- 

 vision, and germ cells are formed, not by con- 

 tributions from all parts of the body, but by 

 division of preceding cells which are derived 

 ultimately from the fertilized egg (Fig. 22). 

 The hen does not produce the egg, but the egg 

 produces the hen and also other eggs. Indi- 

 vidual 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. 



There is a continuity of germinal substance, 

 and usually of germinal cells, from one gener- 

 ation to the next. In some animals the germ 

 cells are set apart at a very early stage of de- 

 velopment, 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 gen- 

 eral then germ cells do not come from dif- 

 ferentiated body cells, but only from un- 

 differentiated germinal cells, and if in a few 

 doubtful cases differentiated cells may reverse 



