MUSEUM OF COMPARATIVE ZOOLOGY. 155 



the micron 11 clear type, inasmuch as the division is in all essential re- 

 spects like that of micronuclei, and in the resting state the nuclei bear 

 no resemblance to macronuclei. The direct division of macronuclei is 

 often accompanied by a longitudinal arrangement of the chromatic fila- 

 ments, resembling that found in the scorpion's serosa (see Figs. 6, 7, 8). 

 It seems to me that Carnoy is wrong in speaking of these longitudinal 

 filaments as a " spindle," for it has never been shown that they converge 

 to the poles of the nucleus, and frequently they can be resolved into 

 granules, which is never the case with spindle fibres. Their resemblance 

 to the spindle of karyokinesis is deceptive. From their behavior with 

 stains, I regard them as consisting of chromatin, and Biitschli ('88, 

 p. 1526) speaks of this stage of the macronucleus as the "Knauelsta- 

 dium," implying that the parallel filaments are chromatic threads. 



Among the Vertebrates, amitosis is unusual, and where it exists kary- 

 okinesis is generally found to occur in cells of the same kind. It is 

 almost confined to cells which do not form fixed tissues, as leucocytes of 

 all kinds, and " giant cells," especially those of the red marrow. It also 

 occurs in testicular cells of Vertebrates. In leucocytes, according to 

 all observers, the nuclear division takes place by constriction, and is 

 freqxaently accompanied by division of the cytoplasm (Eauvier, '75 ; 

 Flemming, '82, p. 344; Ai-nold, '87). But, as the recent work of 

 Flemming ('91) and otliers shows beyond a doubt, leucocytes also di- 

 vide by karyokinesis. It is difficult to say whether there is more than 

 a single kind of leucocyte, one dividing directly, the other indirectly, 

 or whether cells of the same kind divide in two different ways. In 

 case of giant cells, it has been shown by Arnold ('84), Denys ('86), 

 Demarbaix ('89), and others, that division occurs both directly and by 

 multiple karyokinesis. Both kinds of division are followed by division 

 of the cytoplasm, leading to the formation of a brood of daughter cells 

 within the mother cell. 



After going over the literature of amitosis, taking especial note of 

 the manner of its occurrence and distribution in the Animal Kingdom, I 

 have become convinced that it is not derived from mitosis, and, on the 

 other hand, is not the forerunner of the more complicated process. I con- 

 sider it another type of division altogether, which, along with karyoki- 

 nesis, has been transmitted from the simplest forms of life to the most 

 highly organized. While apparently every kind of nucleus may, at 

 some stage of its existence, divide by karyokinesis, many afterwards 

 exchange this type of division for the simpler process. The special 

 conditions which evoke the exchange are very imperfectly understood, 



