300 BIOLOGY OF THE PROTOZOA 



the chromosomes In Paramecium bursar ia. The late stage in the 

 crescent is regarded by her as a spireme from which the chromosomes 

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

 (1907) found that the chromosomes of Paramecium caudatum are 

 derived from a synezesis stage which precedes the crescent and that 

 the chromosomes are already divided at the stage which had gen- 

 erally been regarded as the metaphase. According to this account 

 the metaphase stage occurs during the metamorphosis of the 

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

 early anaphase stage (Fig. 147). 



In other ciliates the chromosomes are formed by the union of 

 chromomeres which are derived by fragmentation of the homogene- 

 ous chromatin of the resting micronucleus. The process is com- 

 pleted at the parachute stage and the definitive number is present 

 by the time the second pole of the spindle is completed. In Urolep- 

 tus mobilis when diffusion of the granules has apparently reached 

 its limit, there are from 16 to 20 chromomeres (48 to 50 in U. halseyi) 



i| i||| (Hi) lllf fl$ 



1 2 3 4 5 



Fig. 150. — Euplotes patella, micronuclear chromosomes. 1, In vegetative mitosis; 

 2, 'A and 4, first, second and third meiotic divisions; 5, first division of the amphi- 

 nucleus. (After Turner, from University of California Publications in Zoology, 1930.) 



(Fig. 32, p. 64). Prandtl's figures show that there are approximately 

 32 in Did i nium nasutum. Enriques (1908) and Collin (1909) have 

 described a similar fragmentation of the comma-shaped chromatin 

 rod <>f Chilodon uncinatus and of the homogeneous chromatin mass 

 of Anoplophrya branchiarum, the granules of chromatin collecting 

 in the center of the first maturation spindle. In Didinium, Chilodon 

 and Anoplophrya these granules fuse until a definite number of 

 chromosomes result— 16 in Didinium (Fig. 148), 4 in Chilodon (8 in 

 the tetraploid form found by MacDougall, 1925, Fig. 149), and 6 in 

 Anoplophrya and 8 in Euplotes patella where each is made up of 

 four previously separated chromomeres (Turner, 1930, Fig. 150). 

 In Uroleptus mobilis a similar fusion of granules results in 8 chromo- 

 somes (Fig. 32, p. 64). Urolejitus halseyi differs in many respects 

 from U. mobilis. Its micronucleus is larger and lacks an endobasal 

 body. The first pole of the first meiotic spindle is formed by con- 

 densation of the karyolymph which draws away from the peripheral 

 chromomeres. The second pole is formed by migration of part of 

 the condensing substance, and between the two poles the nuclear 



