150 BUU-ETIN OF THE 



chromoeomes inrariably inclade aU the stainable substance of the nu- 

 cleos, so that the presence of nudeoh in a nucleus undergoing constric- 

 tion mav be taken as perhaps the strongest evidence of direct division. 

 The behavior of nucleoli in amitosis is of peculiar interest. Where 

 there is a angle nucleolus, it constricts previous to the constriction of 

 the nucleus, according thus with the Eemakian scheme. The division 

 o{ the nucleolus, however, has rarely been observed. It was first de- 

 scribed, I believe, by F. E. Schulze C~5), in the division of Amceba poly- 

 podia; has since been tigiired by Camoy {'So, Plate I. Figs. 10, 12, 13) 

 for Tarioos amitotically dividing Arthropod cells, and by Hoyer ('90) 

 for the intestinal epithelium of EhabdoHema nigroffenotum. A peculiar 

 inodificati<m <rf the nucleolus, and its division into four segments pre- 

 vious to the constriction of the nucleus, was observed by Platner ('89, 

 pp. 115-14r9) in the Malpighian vessels of DiftUeus margmalis. It is 

 eitanemely probable that, whenever the nucleolus is a single and defi- 

 nitely oT^nized structure, it always divides previously to or during con- 

 stricticwi of the nucleus. Where there are several small nucleoli, thev 

 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 

 pb^Mxnena. It takes place by constriction, by formation of division 

 jdanes, by giranmaticHi, and by enlargement of one or more f)erforation8 

 (Arnold, 'S8, Flemming, '89). It is either simple or multiple, and it 

 may or may not be accompanied by division of the ceU. 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 fax the most frequent among vniceUular organifms, amoeboid cell* 

 (leweoqfte$), ajidepUkdial tissues. There seem to be no authentic instances 

 of it in connective tissues (except p<»sibly the fet-cells of Arthropods, 

 described by Camoy). none in nervous tissue, and but one or two in 

 mnade fibres (Camoy, *So, p. 221). Xot only the nuclei of fixed tissues 

 divide by the direct method, but also those of nascent tissues, at least 

 among the Arthmpoda. Direct division is, however, of rare occurrence 

 in the embryo. I believe there are only two authentic instances of 

 it, — that discovered by Camoy in the ventral plate of an embryo of 

 HvdropkUws picen* ('8-5. p. 224, Plate I. Fig. 11), and that found by 

 Wheeler C89, p. 313) in the formation of the blastoderm of Blatta 

 gerwutjuea, where no instance of mitosis was detected. The embryonal 

 monbranes of the scorpion I do not include imder this head, becatise 

 tbcy are tempocazy structures forming no vital pwt of the embryo. 



