730 CALKINS. [Vol. XV. 



since the whole problem remains obscure and requires a thor- 

 ough study with better methods and optical instruments. In 

 sections given above, these fibers which are formed within 

 the nucleus and probably attached to the chromosomes appear 

 to come into close juxtaposition, but not to be continuous with 

 those without, i.e., those seen within and without the nucleus 

 appear to be different from each other, the former originating 

 from the nucleoplasm and the latter from the cytoplasm, just 

 as Brauer thinks concerning the formation of the spindle-fibers 

 of Ascaris megalocephala bivalens" (p. 323). My observations, 

 while not conclusive, show, I believe, that the mantle-fibers 

 (or "radial-fibers" of Ishikawa) are connected with the "nucleo- 

 plasm," but lie completely outside of the nucleus in the sub- 

 stance of the sphere. The mantle-fibers in mitotic figures of 

 most Metazoa are usually first visible just before or during the 

 time of disappearance of the nuclear membrane. So it is with 

 Noctiluca, but here, as in the first stages of spindle-formation 

 in various eggs, the membrane, instead of disappearing entirely, 

 fades away in one part only. 



Much has been written during the last few years on the ori- 

 gin of the spindle-fibers, the main question being whether they 

 are of nuclear or cytoplasmic origin. In the different cells of 

 Metazoa the fibers arise sometimes from the nucleus, sometimes 

 from the cytoplasm. In other cases, i.e., when the mitotic fig- 

 ure contains a central-spindle and mantle-fibers, they arise from 

 both nucleus and cytoplasm, in which case the central-spindle 

 usually comes from the cytoplasm (Hermann ('91), Meves ('96), 

 Flemming ('95), Heidenhain ('94), Ishikawa ('94) )} In many 

 cases, however, no central-spindle can be distinguished, and in 

 such forms the fibers sometimes arise in the cytoplasm (Stras- 

 burger ('92, '97), Mottier ('97), Osterhaut ('97), and botanists 

 in general ; Griffin ('96), Wheeler ('95), Mead ('97), etc.), or, 

 in many cases, from the nucleus (Weismann and Ishikawa ('89), 

 Brauer ('93, '94), Ruckert ('94), Korschelt ('95), Wilson ('96), 

 v. Erlanger ('97) ). 



I have already shown that the fibers of the central-spindle 



' An exception is found in Ascaris megalocephala univalens where the central- 

 spindle is intra-nuclear (Brauer ('93) ). 



