516 BIOLOGY OF THE PROTOZOA 



attracted to the macrogamete until after these fragments are 

 ehminated, but as soon as the granules appear on the surface the 

 microgametes move toward them in the most direct path (loc. cit., 

 p. 257). Zweibainu (1922) observed that the glycogen content is 

 fundamentally diflerent in the two individuals of a conjugating pair 

 of Paramecium, which may be significant in this connection. 



While chemiotaxis may underlie the phenomena described above, 

 an equally intelligible inter])retati()n might be drawn on the basis 

 of differences in potential of a magnetic nature. Two individuals 

 of Uroleptus mohilis about to conjugate, circle about one another, 

 twist and turn but do not become separated; finally they become 

 lightly fused by the extreme anterior parts of their peristomes and 

 the zone of fusion ultimately extends about half way down the peris- 

 tomes. In the early stages, as with Paramecmm , the two individ- 

 uals can be separated without injury to either ("split pairs") but 

 later the two protoplasms are welded into one, forming a proto- 

 plasmic bridge between the individuals. Experiments in cutting 

 apart the two fused indi\'iduals have shown that immediately 

 after contact and initial fusion the complete series of maturation 

 divisions proceeds as though the separated individuals were still 

 in conjugation (Calkins, 1921) and similar cutting at any time during 

 the period of conjugation, does not alter the course of the internal 

 and consecutive processes. Ultimately reorganization of the 

 individual follows in due course and the subsequent happenings 

 are exactly like those of an ex-con jugant. These experiments 

 indicate that the phenomena of maturation and of reorganization 

 which characterize fertilization in Uroleptus mohilis are of the nature 

 of an "all or none" series of reactions and when once started they 

 go through to the end without deviation. It also appears that the 

 stimulus which sets in motion this chain of processes is received at 

 the time of initial contact and is mutually received by both con- 

 jugating individuals. It thus appears to be less of a chemical 

 reaction than a physical one and has many of the attributes of a 

 surface contact phenomenon between surfaces of different electrical 

 potential. 



III. THE PROCESS OF FERTILIZATION. 



The actual process of fusion, with the exception of fertilization by 

 conjugation, furnishes little material for descriptive purposes, two 

 cells come together and fuse, probably with cytolysis of the contig- 

 uous cell membranes. In hologamic forms of ciliates {e. g., in 

 Balatifidiiiut roH according to Brumpt) which are extremely rare, 

 two individuals come together as in pseudo-conjugation of gre- 

 garines; they secrete a common cyst membrane and then fuse 

 completely. 



In isogamic and often in anisogamic fertilization, fusion begins 



