MUSEUM OF COMPARATIVE ZOOLOGY. 149 
E. van Beneden, Boveri, and Watase may be mentioned) are agreed that 
the spindle arises from the cytoplasm. The same view with regard to 
the spindle in the mitosis of vegetable cells was expressed by Stras- 
burger, Guignard, and other botanists. 
The centrosome, as a converging point for the spindle fibres and 
polar rays, plays a most important part in karyokinesis, and, so far as 
known, none at all in amitosis. The centrosome has indeed been found 
by Flemming (91) in leucocytes, which certainly divide amitotically ; 
but there it is a single structure, and as Flemming’s figures show, 
takes no part in the amitotie 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 that 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 formation 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 result 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 
