'840 ORESSWELL SHEARER. 



In the early stages, during the formation of the central 

 spindle, its fibres in part appear to arise outside the area of 

 the nucleus. In one instance I was able to distinguish the 

 spindle-fibres beyond the still evideut remains of the nuclear 

 wall. The centrosome clearly arises beyond the limits of the 

 nucleus, and from the reticulum of the cytoplasm, and its 

 presence can be clearly detected before the dissolution of the 

 nuclear wall. 



Much has been written on the origin of the spindle and the 

 centrosomes as to whether they are of nuclear or c3'toplasmic 

 origin. It has been established that the spindle-tibres may 

 arise from either. In the case of the mantle-fibres they arise 

 almost invariably from the nucleus, while the spindle sub- 

 stance proper arises from the cytoplasm, as has been shown 

 by Meves (22) in Salamandra, Calkins (3) and Ishikawa(17) 

 in Noctiluca, Flemming and Heidenhain (14) in leuco- 

 cytes. In cases where no central spindle is present the 

 astral rays seem to arise from the cytoplasm, as in a number 

 of plants, some worms, as Thalassema, according to Griffin 

 (11), and in a number of Annelids as described by Mead (21). 

 In other cases from the nucleus, according to Flemming (7), 

 Huckert (27), Wilson (33), and Korschelt (18). 



According to Watase (31) the centre of the aster is merely 

 the point where the greatest number of cytoplasmic filaments 

 meet, the centrosome thus produced giving rise in turn to the 

 spindle filaments. Thus the spindle-tibres originate from the 

 centime of the aster, and not from the nucleus. This is clearly 

 shown in the case he instances of the blastomeres of Loligo, 

 where the nucleus remains a clear area in the middle of the 

 central spindle. There is a short period in the formation of 

 the spindle in Histriobdella when almost the same con- 

 ditions ai-e shown. Again, the observations on eggs that 

 have been artificially fertilised by salt solutions clearly point 

 to the origin of the spindle quite independent of the nucleus. 

 According to Wilson (34) all degrees exist between the asters 

 that lie remote from the nucleus and of undoubted cyto- 

 plasmic origin, and those close beside it. 



