THE NUCLEI OF UROLEPTUS MOBILIS 337 



Very few complete studies have been made of conjugation in 

 hypotrichs. Apart from, and since, the classical, epoch-making 

 work of Maupas, who included the full history of conjugation in 

 the hypotrich Onychodromus grandis and a somewhat less com- 

 plete history of Stylonychia pustulata, of Euplotes patella, and 

 E. charon, there has been only one work on hypotrichs, that of 

 Prowazek on Stylonychia pustulata in 1899. 



As is well known, Maupas ('88) described eight (A to H) phases 

 common to all types in the process of conjugation. Phase A, the 

 initial phase, is characterized by the growth and early changes 

 of the micronucleus ; the second phase, B, by the first divis on 

 of the micronucleus. In modern terms this phase is called the 

 first maturation division or first meiotic division. The third 

 phase, C, is the period of the second division, now called the 

 second maturation or second meiotic division. The fourth phase, 

 D, is the period of the third division resulting in the formation 

 of the pronuclei. The fifth phase, E, is the period of interchange 

 and fusion of the pronuclei; the sixth, F, and seventh, G, are 

 the stages of the first and second divisions, respectively, of the 

 fertilization nucleus or amphinucleus. The last phase, H, finally 

 is the period between the second division of the amphinucleus 

 and the first fission of the cell after conjugation. The keen 

 powers of observation and generalization which Maupas pos- 

 sessed are well shown by this recognition of the successive stages 

 in conjugation, particular 1}^ in connection with the distinction 

 between the first and second divisions of the micronucleus, and 

 at a time when the significance of these divisions in maturation 

 phenomena was quite unknown. Subsequent investigations have 

 shown that, with minor variations, these successive phases hold 

 good for the great majority of cases. 



a. The preparatory stage (phase A) of Uroleptus mohilis. The 

 difficulties in working out the initial stages of conjugation in 

 Uroleptus are increased because of the multinucleate condition 

 of the conjugating organisms. The number of micronuclei in 

 vegetative stages varies from four to six and the same variable 

 numbers appear in the conjugating individuals. Analogous con- 

 ditions are found in Paramecium aurelia (Hertwig, Maupas), 



