466 WILSON. [Vol. XI. 



minute to be identified, or destroyed by the reagents, or left 

 unstained ; and, in fact, one or more minute blue granules 

 may sometimes be seen near the center of the aster ; but the 

 study of a very large number of preparations of apparently 

 faultless clearness and sharpness, leads to the conclusion that 

 these granules are inconstant and indistinguishable from other 

 similar granules lying in the surrounding protoplasm. In 

 Echinus Boveri finds sometimes (" nur an einigen besonders 

 gunstigen Praparaten ") a minute deeply staining body in the 

 middle of the sperm-aster, which he identifies with the centro- 

 some, and which he describes as dividing into two just before 

 union of the nuclei. It is impossible at present to determine 

 positively whether this apparent divergence in our results is 

 due to a difference of the methods employed, or to a difference 

 of species. The latter alternative appears to be not improbable, 

 since the arrangement of the astral rays proves that the sperm- 

 aster divides earlier in Echinus than in Toxopneustes {cf. the 

 statements at p. 8 of Boveri's paper with Text-figs. Ill, A B oi 

 this paper, which accurately represent the arrangement of the 

 rays).^ The difference, if it really exist, is, however, of sec- 

 ondary importance, for it relates merely to the morphological 

 configuration of the centrosome, and does not affect the fact 

 that the sperm-aster is developed under the influence of a 

 definite body, derived from the middle-piece of the spermato- 

 zoon, which divides into two to form the dynamic centers of 

 the daughter-cells. 



IV. On the Mechanism of Karyokinesis. 



Van Beneden's beautiful hypothesis of the contractility 'of 

 the spindle-fibres and astral rays has been almost universally 

 accepted by students of the animal cell, and is supported by 

 such an array of well-determined facts — described by himself 

 and Boveri in the case of Ascaris, by Flemming and Hermann 

 in the karyokinesis of amphibian cells, by Solger and Zimmer- 

 man in pigmcnt-cclls, by Heidenhain in leucocytes — that it is 



1 In the star-fish, as in Rhytichelmis, the sperm-aster completely divides before 

 conjugation (Mathews). 



