THE NUCLEI OF UROLEPTUS MOBILIS 341 



their formation or reduction; Enriques ('08) finds the same num- 

 ber in Chilodon uncinatus. Popoff ('08) enumerates sixteen 

 chromosomes in Carchesium polypinum in the first maturation 

 spindle, and Enriques ('07) finds the same number in Opercularia 

 coarctata, while Kaltenbach ('15), somewhat doubtfully, attrib- 

 utes twenty chromosomes to Ophrydium versatile. Collin 

 ('09), finally describes six chromosomal elements in the first 

 spindle of Anoplophrya branchiarum. 



Hamburger ('04) is a bit hazy in her description of the origin 

 of the chromosomes in Paramecium bursaria. The late stage of 

 the crescent is regarded as a spireme from which the chromosomes 

 are formed as short, curved, or V-shaped rods. Calkins and 

 Cull derive the chromosomes of Paramecium caudatum from a 

 synezesis stage which precedes the crescent, and from which the 

 chromosomes emerge as double rods which are fully divided by 

 the time the first spindle is completed. According to this ac- 

 count, the metaphase stage occurs during the metamorphosis of 

 the crescent into the spindle, so that the latter when formed is 

 always in the early anaphase stage. 



The process of chromosome formation in Paramecium caudatum 

 may be regarded as a linear fragmentation of the dense chromatin 

 of the micronucleus. In the other forms in which the chromatin 

 history has been followed, the process may be interpreted as a 

 granular fragmentation, and the problem is to construct the 

 chromosomes from these granules. The parachute nucleus, 

 which is characteristic of most of the ciliates with a small number 

 of chromosomes, may be compared with the Paramecium nucleus 

 at the time when the division center has migrated from the apex 

 of the crescent to the middle of the convex side. The chromatin 

 content of such parachute nuclei consists of separated granules 

 of chromatin, of which the number is difficult to count. In 

 Uroleptus mobilis when diffusion of the granules has apparently 

 reached its limit, I find from twenty-four to twenty-eight such 

 granules (figs. 35 to 37). Prandtl's figures show that there are 

 approximately thirty-two in Didinium nasutum. Enriques ('08) 

 and Collin ('09) describe a similar fragmentation of the comma- 

 form chomatin rod of Chilodon uncinatus and of the homogeneous 



