334? The Unity of the Organism 



tozoa of many animals arises from a basal granule or cen- 

 triole of the spermatid. Illustrations of this may be seen 

 in many of the recent studies of spermatogenesis. If one 

 compares early stages of the transformation of the sper- 

 matid into the spermatozoan, like those of the rat (figure 

 17) with some of Miss jMcCulloch's figure (8) of 

 Crithidia, already described, the resemblance between the 

 two is unmistakable, when one considers the difference in 

 the animal species to which each belongs. In both there is 

 the relatively voluminous cytoplasm, the large nucleus (7^^^c.), 



nuc,- 



FIGURE 17. SPER31ATID Or THE RAT (AFTER DUESBERG). 



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



the filament («./.) connected with the granule (has. gr.), 

 and between this granule and the nucleus another chromatic 

 body (c). To be sure the resemblance falls far short of 

 identity ; but it is nevertheless so striking that hardly any 

 one can avoid recognising it, nor can he well avoid asking 

 what it means. Can it be a resemblance due to descent and 

 hence an instance of heredity.^ That it is due to descent 

 in the meaning of the term as used in our definition of hered- 

 ity is certainly not the case. Descent in that definition 

 means observed descent as when the ancestry of a child 

 is a matter of family record. Such resemblance is declared 

 due to descent because the ancestry is known on other 

 grounds than that of such resemblance as appears be- 

 tween the adult unicellular Crithidia and the developing 



