MUSEUM OF COMPARATIVE ZOOLOGY. 149 



E. van Beneden, Bove'ri, and Watase may be mentioned) are agreed that 

 the spiudle arises from the cytoplasm. The same view with regard to 

 the spiudle in the mitosis of vegetable cells was expressed by Stras- 

 burger, Guiguard, and other botanists. 



Tlie ceutrosome, as a converging point for the spindle fibres and 

 polar rays, plays a most important part in karyokiuesis, and, so far as 

 known, none at all in amitosis. The ceutrosome has indeed been found 

 by Flemming ('91) in leucocytes, which cei'tainly divide amitotically ; 

 but there it is a siugle structure, and as Flemming's figures show, 

 takes no part in the amitotic division of the nucleus. Whether it also 

 remains passive during the mitotic division of leucocytes and in amitosis 

 followed by division of the cell, is not known. It has been supposed by 

 Carnoy ('85) that spindle rays were present in certain nuclei which 

 divide amitotically, but this seems extremely doubtful, especially since 

 they have no perceptible action on the chromatic substance. I believe 

 it can be shown in every case of amitosis known, that the division of 

 the chromatin is accomplished independently of chromosomes, spindle rays, 

 or any other visible influence outside of the nucleus. 



The persistence of the nuclear membrane in amitosis, and its dis- 

 appearance in mitosis, were formerly considered points of distinction 

 between the two kinds of division ; but, as is well known, more recent 

 studies have shown tliat the membrane persists in many cases of un- 

 doubted karyokinesis, especially among the Arthropods (Carnoy, '85) and 

 Protozoa (Gruber, '83, R. Hertwig, '84, Pfitzner, '86'', and Schewiakoff, 

 '88). Its presence seems to offer no obstacle to the karyokinetic 

 changes, and Watase ('91) has pointed out that it need not prevent 

 the foi'mation of an extra-nuclear spindle, the rays of which may pene- 

 trate the membrane. In the nuclei of Opalina ranarum, and in the 

 micronuclei of Infusoria generally, where, according to all observers, the 

 nuclear membrane persists, the mitotic division is accompanied by con- 

 striction ; but the fact that constriction is here visible may be considered 

 as in some measure a residt of the persistence of the membrane, thereby 

 making evident the outline of the karyoplasm. Yet constriction does 

 not always take place when the membrane persists, for in the spermatic 

 cells of Pagurus striatus, figured by Carnoy ('85, Plate VII. Fig. 244), 

 the nuclear membrane is visible at all stages, and gives no evidence of 

 constriction. 



The modification of the chromatic substance into chromosomes is 

 usually the most conspicuous feature of karyokinesis, and in most cases 

 serves to distinguish mitotic nuclei from any of the amitotic ones. The 



