224 University of California Publications in Zoology [Vol. 19 



then pushed by this expanding membrane out to the periphery of 

 the nucleus, where it reposes in a niche of the nuclear membrane, but 

 separated from the mitotic area within by the persistent kinetic 

 membrane (pi. 11, figs. 38, 44). 



It has been found on several occasions (pi. 11, fig. 43) to break up 

 and form an intranuclear chromatin cloud. Even where it has been 

 traced repeatedly to its position near the nuclear membrane, I have 

 never located it or any similar chromatin mass outside the nucleus. 

 In some instances it seems that the cytoplasm just around the nucleus 

 is more deeply stained by a chromidial cloud as though the macro- 

 karyosome or a part of it were extruded in fine granules. Such is 

 undoubtedly its fate in many individuals. But much evidence points 

 to the gradual absorption of the macrokaryosome in situ in some 

 instances within the nuclear membrane, with the formation of an 

 intranuclear, intrakinetic membrane chromatin cloud (pi. 10, figs. 35, 

 36, pi. 11, figs. 37, 38, 41, 42, 44, 45). There is some evidence (pi. 11, 

 fig. 46, pi. 12, fig. 48) that it may in other instances persist and pass 

 over to one of the daughter nuclei without complete dissolution. 

 There is no evidence for its splitting or division on the equatorial 

 plate. Its behavior seems dependent in some way upon the meta- 

 bolic equilibrium of the nucleus and cytoplasm. Its significance, 

 regarding possible relations with the parabasal body of parasitic 

 flagellates, the macronucleus of ciliates, Hertwig's theory of tropho- 

 chromatin and idiochromatin, and Hartmann 's binuclear theory, in 

 so far as flagellates are concerned, will be taken up in subsequent 

 discussion. 



Individuals iu evident stages of the prophase have been observed 

 in which there is a total absence of any evidence of a constriction 

 of the karyosome. The kinetic karyosome (microkaryosome) entering 

 into mitosis may likewise on rare occasions be larger than the other 

 mass, the passive macrokaryosome. 



Mitosis 



With the organization of the microkaryosome mitosis begins. 

 Binary fission by longitudinal division seems to be initiated within 

 this karyosome. This forms about itself the kinetic membrane, which 

 continues to expand until it becomes commensurate with the nuclear 

 membrane. The faint rhizoplasts extend from the kinetic membrane 

 which surrounds the organizing microkaryosome, through the 



