134 PROTOZOOLOGY 



As to the origin and development of the achromatic figure, vari- 

 ous observations and interpretations have been advanced. Certain 

 Hypermastigina po.ssess very large filiform centrioles and a large 

 rounded nucleus. In Barbulanympha (Fig. 60), Cleveland (1938a) 

 found that the centrioles vary from 15 to 30/x in length in the four 

 species of the genus which he studied. They can be seen, according 

 to Cleveland, in life as made up of a dense hyaline protoplasm. 

 When stained, it becomes apparent that the two centrioles are 

 joined at their anterior ends by a desmose and their distal ends 20 to 

 30m apart, each of which is surrounded by a special centrosome (a). 

 In the resting stage no fibers extend from either centriole, but in the 

 prophase, astral rays begin to grow out from the distal end of each 

 centriole (6). As the rays grow longer (c), the two sets soon meet and 

 the individual rays or fibers join, grow along one another and over- 

 lap to form the central spindle (d). In the resting nucleus, there are 

 large irregular chromatin granules which are connected by fibrils 

 with one another and also with the nuclear membrane. As the achro- 

 matic figure is formed and approaches the nucleus, the chromatin be- 

 comes arranged in a single spireme imbedded in matrix. The spireme 

 soon divides longitudinally and the double spireme presently breaks 

 up transversely into paired chromosomes. The central spindle begins 

 to compress the nuclear membrane and the chromosomes become 

 shorter and move apart. The intra- and extra-nuclear fibrils unite as 

 the process goes on (e), the central spindle now assumes an axial 

 position, and two groups of V-shaped chromosomes are drawn to 

 opposite poles. In the telophase, the chromosomes elongate and be- 

 come branched, thus assuming conditions seen in the resting nucleus. 



In the unique resting nucleus of Spirotrichomjmpha polygyra (Fig. 

 61), Cleveland (1938) found four chromosomes, each of which con- 

 tains a distinct coil within a sheath and its one end connected with 

 the anterior margin of the nuclear membrane by an intranuclear 

 chromosomal fiber, and the other end with a deeply staining endo- 

 some (a). The spindle fibers appear between the separating flagellar 

 bands which come in contact with the nuclear membrane. Soon 

 some of the astral rays become connected with the intranuclear 

 chromosomal fibers and one long and one short chromosomes which 

 become thicker and shorter move toward each pole. During the telo- 

 phase, each chromosome splits lengthwise and forms the resting 

 nucleus (g). 



In Lophomonas hlattarum, the nuclear division (Fig. 62) is initiated 

 by the migration of the nucleus out of the calyx. On the nuclear 

 membrane is attached the centriole which probably originates in the 



