94 HEREDITY AND ENVIRONMENT 



ferentiated somatic cells, but only from un- 

 differentiated germinal cells, and if in a few 

 doubtful cases differentiated cells mav reverse 



FIG. 22. DIAGRAM SHOW- 

 ING THE "CELL LINEAGE" OF 

 THE BODY CELLS AND GERM 

 CELLS IN A WORM OR MOL- 

 LUSK. The lineage of the 

 germ cells ("germ track") 

 is shown in black, of ecto- 

 derm in white, and of endo- 

 derm and mesoderm in 

 shaded circles. The whole 

 course of spermatogenesis 

 and oogenesis is shown in 

 the lower right of the figure 

 beginning with the primitive 

 sex cells (Prim. Sex Cells) 

 and ending with the game- 

 tes, the genesis of the sper- 

 matozoa being shown on the 

 left and that of the ova on 

 the right. 



