150 BULLETIN OF THE 
chromosomes invariably include al the stainable substance of the nu- 
cleus, so that the presence of nucleoli in a nucleus undergoing constric- 
tion may be taken as perhaps the strongest evidence of direct division. 
The behavior of nucleoli in amitosis is of peculiar interest. Where 
there is a single nucleolus, it constricts previous to the constriction of 
the nucleus, according thus with the Remakian scheme. The division 
of the nucleolus, however, has rarely been observed. It was first de- 
scribed, I believe, by F. E. Schulze (’75), in the division of Ameba poly- 
podia; has since been figured by Carnoy (’85, Plate I. Figs. 10, 12, 13) 
for various amitotically dividing Arthropod cells, and by Hoyer (’90) 
for the intestinal epithelium of Zrhabdonema nigrovenosum. A peculiar 
modification of the nucleolus, and its division into four segments pre- 
vious to the constriction of the nucleus, was observed by Platner (’89, 
pp. 145-149) in the Malpighian vessels of Dytiscus marginalis. It is 
extremely probable that, whenever the nucleolus is a single and defi- 
nitely organized structure, it always divides previously to or during con- 
striction of the nucleus. Where there are several small nucleoli, they 
may indeed arrange themselves so as to be equally apportioned to the 
daughter nuclei ; but they are not known to divide, as the chromosomes 
in mitosis do. 
Amitotic division, even more than karyokinesis, is variable in its 
phenomena. It takes place by constriction, by formation of division 
planes, by gemmation, and by enlargement of one or more perforations 
(Arnold, ’88, Flemming, ’89). It is either simple or multiple, and it 
may or may not be accompanied by division of the cell. The resulting 
nuclei may be equal or unequal. Amitosis occurs throughout both the 
Animal and Vegetable Kingdoms ; but as far as animals are concerned, 
it is far the most frequent among unicellular organisms, amaboid cells 
(leucocytes), and epithelial tissues. There seem to be no authentic instances 
of it in connective tissues (except possibly the fat-cells of Arthropods, 
described by Carnoy), none in nervous tissue, and but one or two in 
muscle fibres (Carnoy, ’85, p,221). Not only the nuclei of fixed tissues 
divide by the direct method, but also those of nascent tissues, at least 
among the Arthropods. Direct division is, however, of rare occurrence 
in the embryo. I believe there are only two authentic instances of 
it, — that discovered by Carnoy in the ventral plate of an embryo of 
Hydrophilus piceus (85, p. 224, Plate I. Fig. 11), and that found by 
Wheeler (89, p. 313) in the formation of the blastoderm of Blatta 
germanica, where no instance of mitosis was detected. The embryonal 
membranes of the scorpion I do not include under this head, because 
they are temporary structures forming no vital part of the embryo. 
