FERTILIZATION 631 



which point they fuse in pairs, forming diads before entering upon the 

 resting stage. 



A tetraploid strain arose spontaneously in a pure culture of Mac- 

 Dougall's Chilodonella uncinalus. Maturation phenomena in the tetra- 

 ploid form were similar to those of the usual diploid form, except that 

 there were twice the number of chromosomes in every stage. Investiga- 

 tions, presented and reviewed in a later article by MacDougall (1936), 

 show meiotic processes which are similar in six species of Chilodonella. 

 In all species the diploid number of chromosomes is four. A parachute 

 stage is followed by the formation and the synaptic pairing of chroma- 

 tin threads, as in "classic leptotine and zygotine" stages, which condense 

 to form the pachytene chromosomes. MacDougall's descriptions reveal 

 the striking similarity of meiosis in Chilodonella to the general scheme 

 of meiosis in the Metazoa. Messiatzev (1924) reported synapsis occur- 

 ring in the first maturation division and again in a fifth amphinuclear 

 division of Lionotus lamella, but Poljansky (1926) believes that Mes- 

 siatzev confused his stages in the latter case. 



The small number and the large size of the chromosomes in Chilodo- 

 nella make this a very favorable form for study of meiotic phenomena. It 

 seems unfortunate that more of the recent studies that have genetic sig- 

 nificance were not made on this animal, in which chromosome behavior 

 is clear-cut and well known, instead of on Varamecium, in which it is 

 practically impossible to determine any of the significant stages in 

 meiosis. 



STAGE C, THE SECOND MEIOTIC DIVISION 



The second meiotic division is the reduction division in all ciliates 

 thus far studied, except in Oxytricha jallax, according to Gregory 

 (1923). Prandtl (1906), in his work on Didinium nasutum, was the 

 first to present conclusive evidence on chromosome reduction in ciliates 

 when he described reduction from sixteen to eight chromosomes in the 

 second meiotic division. There seems to be no general rule for the num- 

 ber of nuclei that enter this division. In the species of Chilodonella 

 studied by MacDougall (1936), only one nucleus is involved in any of 

 the three progamous divisions; the other products degenerate. 



In perhaps the majority of ciliates both products of the first division 

 enter the second division. In O. jallax (Gregory, 1923) and in forms 



