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 but that this group presents the most primitive types of 

 nuclear division. So far as known, the very lowest forms of animal cells 

 (Amcebce) always divide by the direct method, as the study of Amoeba 

 poll/podia by F. E. Schulze ('75), and of Pelomyxa villosa, Amoeba secunda, 

 and A. proteus by Gruber ('83 and '85), has shown. The division of the 

 nucleus oi Amoeba p)roteus 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 occtu's. 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 Ainoeba proteus must be relegated to amitosis. The presence of 

 so perfect a type of karyokinesis as that found in Euglypha 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 

 gevimipara, 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 



