BLACKMAN: THE SPERMATOGENESIS OF SOOLOPENDUA. 105 



sidering these diametrically opposite conclusions it must be remem- 

 bered, as Mead ('98) has painted out, that one is based upon undisputed 

 observed facts, while the other is based upon negative evidence. " To 

 maintain that the centrosomes are absent simply because they are not de- 

 monstrable is, of course, to base an assumption upon negative evidence, 

 — a procedure especially dangerous when applied to the centrosome, 

 inasmuch as this structure is, at best, very minute and comparatively 

 difficult to demonstrate, even when its exact position is indicated by 

 the presence of an aster." 



By far the hardest blow to the theory of the continuity of the centro- 

 some has been given by the new school of experimental cytologists. 

 The experiments of Hertwig ('96), Morgan ('96), and Wilson (:01 a 

 and :01 b ) upon Echinolerm eggs treated with solutions of various salts 

 and poisons seem to have proved beyond doubt that functional centro- 

 somes may arise de novo in unfertilized eggs. It is not my purpose here 

 to discuss these experiments ; but one cannot read the recent experi- 

 mental work of Wilson (:01 a ) without being convinced that centrosomes 

 really may arise either in close relation to the nucleus or free in the 

 cytoplasm. Under the same general head may be classed the observa- 

 tions of Mead \ipon the egg of Chaetopterus. He finds that Avhen the 

 cg'^ is removed from the body fluid and placed in sea-water a number of 

 asters soon appear. These asters gradually fade away, with the excep- 

 tion of two, which persist and increase in size. Small bodies appear in 

 their centres which serve as the maturation centrosomes. At first glance 

 these observations would seem to disprove the universal persistence of 

 the centrosomes, but Wilson (:01 b ) assures us that this is not necessarily 

 true. While the centrosome may not persist as such, it is possible that 

 centrosome substance capable of producing a centrosome is present in 

 the cell at all times, from which, as occasion demands, centrosomes may 

 ai ise apparently de novo. 



These experimental studies have left the question of the origin of the 

 centrosome in such an unsettled state that I believe a further discussion 

 at this time is useless. No universal conclusions can as yet be drawn 

 concerning the rank to be accorded to this structure among the organs 

 of the cell. In the testicular cells of Scolopendra heros the conclusion 

 would seem to be justifiable that the centrosome is as truly a cell organ 

 as is the nucleus. A similar conclusion has been reached in many other 

 cells as well ; but the evidence to the contrary brought forward by Lillio 

 and others, and the even more conclusive evidence furnished by the 

 experimental work of Hertwig, Morgan, and Wilson, should also be 



