The Unity of the Organism 



of the same origin. While, as stated above, it may be ac- 

 cepted as proved that in some animals the centrosome arises 

 from the nucleus, in a far larger number of animals the 



FIGURE 18. SPERMAT1D OF SALAMANDER (AFTER HEIDENHAlx). 



a.f., axial filament, c., centrosome. nuc., nucleus. 



evidence is all against such an origin. Furthermore, in many 

 animals a pair of "cromioles" occur, seemingly homologous 

 to t]je chromosomes, situated in the cytoplasm of various 



FIGURE 19. SPERIMATID OF SNAIL (AFTER HEIDENHAIN ). 



cells at a point as remote as possible from the nucleus; and 

 from one of these in the sperm mother cell the axial filament 

 of the spermatozoan grows out. 



The development of the spermatozoan from the spermatid 

 in two widely separated animals, a salamander (figure 18, 

 a and b) and a land snail (figure 19)> illustrates several 

 interesting aspects of the point before us. In both the sper- 



