340 GARY N. CALKINS 



h. First maturation spindle and the chromosome problem. In 

 the conjugation of ciliates with multiple micronuclei the usual 

 result of the prophase activities is the formation of one first 

 maturation spindle for each of the micronuclei present. In 

 Onychodromus grandis each of the two micronuclei forms a 

 spindle (Maupas, '88). The same is true of Stylonychia pustulata 

 (Maupas, '88; Prowazek, '99), and Paramecium aurelia (Hert- 

 wig, '89). In Didinium nasutum (Prandtl, '06), and in Blephar- 

 isma undulans (Calkins, '12), if more than two micronuclei are 

 present, only two of them form spindles; in Bursaria truncatella, 

 on the other hand, all of the sixteen to eighteen micronuclei are 

 active at this stage (Prowazek, '99). In Uroleptus mobilis 

 there may be two, three, or four of these primary spindles (figs. 

 45 to 47). 



The chief interest of these first maturation nuclei lies in the 

 so-called chromosomes, especially in the methods of their for- 

 mation and division. Unfortunately, we have but little exact 

 knowledge on these points owing to the frequent large numbers 

 and small size of the chromatin elements. 



Maupas ('88) made no attempt to enumerate the chromosomes; 

 nor did he describe their formation beyond the brief account of 

 the fragmentation of the homogeneous chromatin masses of the 

 micronuclei. Hertwig ('89) believed that there were eight or 

 nine chromosomes in Paramecium aurelia, basing his view not on 

 the chromosomes, but on the number of fibers which he could 

 distinguish in the connecting strand between the two daughter 

 nuclei. Later observers find that the number, in all species of 

 Paramecium, is very much greater than this (more than 100 

 (Hamburger, Calkins, and Cull) ). 



In some more favorable types than Paramecium the number 

 of chromosomes has been made out with some degree of accuracy. 

 Prandtl ('06) finds sixteen in Didinium nasutum, a number which 

 I have been able to confirm (unpublished). Prowazek ('99) was 

 a little in doubt whether there were twelve or thirteen in the nu- 

 clei of Bursaria truncatella, but describes six chromosomes in 

 Stylonychia pustulata. Stevens ('10) describes four chromosomes 

 in Boveria subcylindrica, var. concharum, but gives no details as to 



