REPRODUCTION 157 



Naville), various Hypermastigina (Kofoid; Duboscq and Grasse; 

 Kirby; Cleveland and his associates). 



In numerous species the division of the centriole (or blepharo- 

 plast) and a connecting strand between them, which has been called 

 desmose (centrodesmose or paradesmose), have been observed. Ac- 

 cording to Kofoid and Swezy (1919), in Trichonympha campanula 

 (Fig. 60), the prophase begins early, during which 52 chromosomes 

 are formed and become split. The nucleus moves nearer the anterior 

 end where the centriole divides into two, between which develops a 

 desmose. From the posterior end of each centriole, astral rays extend 

 out and the split chromosomes form loops and pass through "tangled 

 skein" stage. In the metaphase, the equatorial plate is made up of 

 V-shaped chromosomes as each of the split chromosomes is still 

 connected at one end, which finally becomes separate in anaphase, 

 followed by reformation of two daughter nuclei. 



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

 ous observations and interpretations have been advanced. Certain 

 Hypermastigina possess very large filiform centrioles and a large 

 rounded nucleus. In Barbulanympha (Fig. 61), Cleveland (1938a) 

 found that the centrioles vary from 15 to 30^ 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 

 30/x 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. 



