150 BULLETIN OF THE 



chromosomes invariably include all the stainable substance of the nu- 

 cleus, so that the presence of nucleoli in a nucleus uudergoing 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 Kemakian 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 Amoeba 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 Rhahdonema nigrovenostim. 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 tlie 

 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 vnicellular organisms, amoeboid 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 

 Hydro}yhilus piceus ('85, p. 224, Plate I. Fig. 11), and that found by 

 Wheeler ('89, p. 313) in the formation of the blastoderm of Blatta 

 germanicn, where no iustauce 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. 



