390 The Unity of the Organism 



different genera and species of the group. The very es- 

 sence of the conception of heredity, i.e., resemblance due to 

 genetic kindred, is obvious in the reappearance of a particu- 

 lar type of epimerite in every generation of a given species, 

 while in the group as a whole there is so great a variety of 

 types. 



(i) The ^'Division Center*' of Noctiluca 



Thus far in this section we liave been considering the 

 contribution of non-chromatic substances to the production 

 of permanent organs in the protozoan body, mostly to or- 

 gans by which the animals maintain their relation to the 

 external world. We will now examine a few cases wherein 



FIGURE 34. NOCTILUCA MILIARIS ( AFTER HERTWIG). 



n., nucleus, o., mouth, f., filament, t., tentacle. 



such substances play a leading role in propagation. Per- 

 haps the most striking example is furnished by the well- 

 known marine protozoan Noctiluca (figure 34). To state 

 the point as compactly as possible, division in this animal 

 is started off and led throughout by a large, dimly staining 

 body situated in the cytoplasm adjacent to the nucleus. The 

 division of the nucleus seems to be a process attendant upon, 

 and probably dependent upon tlie division of this body, 

 known as the "division center." (Fig. eS5a a.sp.) 



