MUSEUM OF COMPARATIVE ZOOLOGY. 539 



the other hand, be anticipated if the moving body is approaching the 

 source of the moving power. But if this should be proved inconsistent 

 with other phenomena accompanying nuclear division, it might be possi- 

 ble to refer the retardation to an increasing opposition to motion offered 

 by the substance in the vicinity of the astral centre. 



The slender threads which remain behind the separating lateral zones 

 of thickenings, I have called, not to prejudge in the use of a name, inter- 

 zonal filaments. If the spindle fibres are, as has been generally maintained, 

 composed exclusively of nuclear substance, it is not apparent what these 

 interzonal filaments may be. It has been believed by some observers 

 that they represented a kind of product of the activity of the nuclear 

 substance of the fibres. It has been claimed that their substance, 

 like that of the thickenings, is ultimately employed in the growth of the 

 new nuclei, and that consequently they are nuclear substance. That 

 they differ from the spindle fibres and the varicosities is shown by their 

 not staining as intensely as the latter, and by the fact that they do not 

 as promptly respond, to the force which causes the migration of the 

 stainable substance. While the evidence is too strong to allow a doubt 

 as to their being partly consumed in the growth of the new nuclei, I 

 think there is sufficient proof that the whole of their substance is not in- 

 corporated in the nuclei. The evidence in Limax (Figs. 29, 80", and 91) 

 is as satisfactory as could be expected in cases where the changes cannot 

 be directly observed. Neither their deportment with reagents * nor 

 their fate compels the belief that they are nuclear substance. I am 

 therefore disposed to believe that they are in composition like the 

 interstellate rays at their inception, — that they are, in other words, the 

 spindle fibres deprived of their nuclear substance, and that they differ 

 from the vitelline protoplasm with which they ultimately coalesce only 

 in their greater compactness and refringency. 



Origin of Nuclei. — The formation of nuclei in early stages of on- 

 togeny results from a fusion of nuclear substance with protoplasm, and 

 occurs under two slightly modified forms. The production of the female 

 pronucleus is like that which takes place at each segmentation, and offers 

 no peculiarities capable of supplementing the knowledge of the process 

 which one may acquire from ordinary cell division ; but the origin of the 

 male pronucleus occurs under such different circumstances, that the 

 method of its formation throws additional light on the nature of nuclear 

 production. 



* Compare Flemming's statement, p. 360. 



