MUSEUM OF COMPARATIVE ZOÖLOGY. 155 
the micronuclear 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 Bütschli (88, 
p. 1526) speaks of this stage of the macronucleus as the “ Knäuelsta- 
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 
frequently accompanied by division of the cytoplasm (Ranvier, ’75; 
Flemming, ’82, p. 344; Arnold, ’87). But, as the recent work of 
Flemming (91) and others 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, 
