154 BULLETIN OF THE 
are to be explained in this way, and such an explanation seems to apply 
well, as suggested on a preceding page, in the case of the scorpion’s 
serosa. Division of the cell does not follow as a rule, and upon this fact 
Chun lays stress. But, so far as we know, there is nothing to exclude 
the subsequent occurrence of cell division, and it is even probable that 
cell division is induced by the presence of more than one nucleus. This 
I take to be the case in the scorpion’s serosa, where I believe the division 
of the cell is due in part to the dicentricity set up in the cytoplasm by 
the division of the nucleus. 
The study of nuclear division among the Protozoa seems likely to 
throw much light upon the relations of amitosis to mitosis, for there can 
be little doubt bat that this group presents the most primitive types of 
nuclear division. So far as known, the very lowest forms of animal cells 
(Amebe®) always divide by the direct method, as the study of Amaba 
polypodia by F. E. Schulze (75), and of Pelomywxa villosa, Amoeba secunda, 
and A. proteus by Gruber (’83 and ’85), has shown. The division of the 
nucleus of Amaba proteus takes place by a sharp equatorial cleft, passing 
through the large, centrally placed nucleolus, and dividing that and the 
peripheral zone of chromatin into two exactly equal halves, which after- 
wards move apart. This is regarded by Gruber (’83, p. 385) as a simple 
type of karyokinesis, because an exact division of the chromatin is accom- 
plished. No kinetic change of the chromatic substance is necessary to 
bring this about, hence none occurs. It seems to me that the absence 
of centrosomes and a spindle effectually separates this type of division 
from true karyokinesis, and until these are discovered, the nuclear di- 
vision of Ameba proteus must be relegated to amitosis. The presence of 
so perfect a type of karyokinesis as that found in Luglypha alveolata, 
worked out so completely by Schewiakoff (’88), is strong evidence against 
the hypothesis that karyokinesis was gradually evolved from direct di- 
vision. For here, among the lowest forms of animal life, we have nuclei 
dividing both by a simple constriction, and by the most highly developed 
kinetic changes. 
Nuclear division among the Infusoria is of special interest, for we 
regularly find in the same individual nuclei very different in structure 
and function, — macro- and micronuclei. The former divide directly, 
the latter by karyokinesis. Apparent exceptions are seen in Spirochona 
gemmipara, where, according to R. Hertwig (’77) the macronucleus 
divides by karyokinesis ; and in Opalina ranarum, studied most carefully 
by Pfitzner (86>). As only one kind of nucleus is found in Opalina, 
it is probable, as Biitschli suggests (’88, p. 1500), that these are of 
