254 University of California Publications in Zoology [Vol. 19 



(pi. 11, fig. 47), these being fused end to end into half the number on 

 the equatorial plate. It is, then, these doubled chromosomes which 

 separate transversely in the metaphase. A third alternative is that 

 the chromosomes do split transversely, the inheritable characters being 

 usually halved physiochemically but not necessarily according to the 

 chiasma-type hypothesis. 



In behavior, at least, there are two kinds of chromatin. That of the 

 macrokaryosome is largely passive, that of the microkaryosome is either 

 active or activated by a close association with the division center. The 

 former may possibly be analogous to the macronucleus of ciliates 

 and the parabasal body of parasitic flagellates; the latter to the 

 micronucleus of ciliates and the typical nucleus of flagellates. The 

 distinction of trophochromatin and idiochromatin might be applied 

 here as well as in any of the typical usually cited instances. The 

 chromatin may be all of like character, its behavior being determined 

 in all cases by associated elements. Its close association with achro- 

 matic elements and its inclusion in all chromosomes is, therefore, 

 essential. 



GENERAL SUMMARY 



1. The first evidence of mitosis is an unequal constriction and 

 differential division of the primary karyosome of the vesicular nucleus 

 into a macrokaryosome and a microkaryosome, the latter alone func- 

 tioning directly in the formation of the prophase skein. 



2. The skein originates by the successive segmentation of the micro- 

 karyosome, resulting in two crescents and four terminal knobs. 



3. These crescents split longitudinally, producing presumably eight 

 terminal knobs which are the elements at least of chromosomes. It is 

 possible that one of the terminal knobs fails to split. In this case the 

 number of chromosomes would be seven, which coincides with the best 

 count so far made. 



4. Coincident with the beginning of the segmenting skein, there is 

 organized around the microkaryosome a kinetic membrane which 

 expands until it becomes apparently commensurate with the nuclear 

 membrane. 



5. In the final prophase there is a precocious splitting of the seg- 

 mented skein, in which the number of terminal chromatin masses is 

 doubled, and all are organized in an equatorial belt. These are prob- 

 ably fused in telosynapsis. 





