Reproduction and Life-Cycles 



63 



nuclei supposedly containing no interphase "chromatin granules," has 

 been reported in certain Protozoa. In some of these, such as Endolimax 

 nana, periendosomal material has been demonstrated in more recent in- 

 vestigations (240). Furthermore, Noble (200) reports a functional separa- 

 tion of endosomal granules and periendosomal chromatin in Entamoeba 

 gifigwalis, in spite of their intermingling during early prophases. How- 

 ever, the interphase precursors of the chromosomes have not yet been 

 identified with certainty in many Protozoa and much remains to be 

 learned about the earliest stages of mitosis in most species. 



The observations of Cleveland (63) on Holoniastigotoides have shown 

 that the chromosomes persist as such throughout the mitotic cycle. The 

 diagrammatically clear behavior of the chromosomes in this genus sup- 

 j3lies a logical pattern for interpreting mitosis in smaller nuclei. Each 

 chromosome consists of a coiled chromoyiema embedded in a matrix. Only 

 the matrix is distinguished in heavily stained preparations, but both com- 

 ponents can be detected with phase-contrast microscopy and also by 

 ordinary microscopy in suitably stained preparations. The disappearance 

 of major coiling and the apparent lengthening of each chromonema, as 

 the chromosomal matrix disappears late in mitosis, result in long twisted 

 filaments. This stage, in small nuclei containing a nvnnber of chromo- 

 somes, would suggest the interphase "reticulum" described in various 

 species. If the uncoiled chromosomes are very slender, optical sections of 

 a small nucleus might suggest a granular organization of the chromatin. 

 Origin of chromosomes from such a "granular" or "reticular" interphase, 

 in the light of chromosomal behavior in Holomastigotoides, may involve 

 only a condensation of preexisting chromosomes. Each chromonema be- 

 comes more and more tightly coiled (in the "major coils" of Cleveland) 

 as a new matrix is developed. The result is the more or less compact 

 chromosome of the later prophases. According to Cleveland, duplication 

 of each chromonema occins before the development of the new matrix. 



Eumitosis and paramitosis 



Differences in chromosomal behavior and the structure of the 

 achromatic division-figure have formed the bases for various classifications 

 of protozoan mitoses. Among these systems, that of Belai- (20) has the 

 advantage of simplicity in recognizing two general types, eumitosis and 

 paramitosis. 



Characteristic features of eumitosis are longitudinal splitting of the 

 chromosomes, the development of compact prophase chromosomes, and 

 the appearance of an equatorial belt of chromosomes within the spindle. 

 Many protozoan mitoses can be fitted into such a scheme. 



Nuclear division in Dimorpha mutans (Fig. 2. 6) is representative. 

 Mitosis is initiated by division of the centrosome, and the subsequent 

 development of an amphiaster is accompanied by the formation of short 



