V 



Chromatm as ^"Hereditary Substance^* 337 



matid possesses a . lasli or flagelluin wliich arises from the 

 outermost of a ]5air of <rranules or centrioles situated in the 

 cytoi)]asm just beneath the surfaee of tlie celL In an early 

 stage of the transformation of the salamander spermatid the 

 pair of centrioles moves inward toward the nucleus, the inner 

 member of the pair finall\' entering the nucleus and becoming 

 the middle piece of the sperm head, while the outer membrane 

 is converted into a ring which finally contributes the undulating 

 membrane of the tail of the completed sperm. 



Connection between the centrioles and nucleus in the snail's 

 sperm is accomplished in a different manner. Instead of an 

 inward migration of the pair as in the salamander, the inner 

 member of the pair sends an ingrowth toward the nucleus, the 

 centrioles themselves remaining at the surface of the cell and 

 remote from the nucleus. This ingrowth becomes much elon- 

 gated and produces finally the axial thread of the sperm, the 

 anterior end of which is embedded in the nuclear part of the 

 head. Much this same sort of thing occurs in many other 

 animals, both vertebrate and invertebrate. 



There can be no question then that in a large number 

 of animals the centriole of the sperm is primarily quite inde- 

 pendent of the nucleus, and only becomes connected with it 

 as the sperm develops. Consequently, to speculate that orig- 

 inally or ancestrally the nucleus gave rise to the centriole 

 and axial thread of the sperm is to go exactly contrary to 

 the most direct and positive evidence we have bearing 

 on the question. To this direct evidence drawn from the 

 study of spermatogenesis, that the centriole is in its origin 

 quite independent of the nucleus, should be added the ex- 

 tensive evidence that the centriole is self-propagating by 

 division and passes on from cell generation to cell generation 

 somewhat as the nucleus does. But this fact is so familiar 

 a part of elementary cytology as to need no special treat- 

 ment. 



The upshot of this discussion is that while as regards 

 flagella in some protozoa there is solid observational gi'ound 

 on which to rest the theory that chromatic bodies of the 



