FERTILIZATION 621 



jugating zooids is formed, resembling superficially a double tapeworm. 

 After a time the fissions cease, and conjugation proceeds between mem- 

 bers of each pair of zooids according to the "classical scheme," although 

 the nuclear details have not been worked out. Eventually fission is com- 

 pleted, and the exconjugants soon separate, reorganizing themselves in 

 the usual way. These fissions of the paired conjugants appear to be 

 related in kind to the special preconjugation fissions, observed in several 

 other ciliates, which result in conjugants that may be distinguished from 

 the vegetative forms chiefly by their smaller size. Specialized conjugants 

 are observed in Nicollella cteriodactyli and Collinella gundi'i (Chatton 

 and Penard, 1921); the Ophryoscolecidae (Dogiel, 1925); D He plus 

 gigas (Visscher, 1927); Balantidium sp. (Nelson, 1934); Nyctotherus 

 cordiformis (Wichterman, 1937), and in the microgametes of peri- 

 trichs. 



The preliminary division of the micronucleus in Euplotes charon and 

 E. patella (Maupas, 1889; Turner, 1930), without fission of the body, 

 is a modification of this same tendency of the conjugants to differ from 

 the vegetative forms. 



Conjugants of many other ciliates are in some degree smaller than 

 vegetative individuals, and this may be the result of reduced feeding or 

 of other factors as yet unknown. It is among the copulating ciliates that 

 the greatest difl^erence occurs between the vegetative forms and the 

 mature gametes (see p. 610 above) . 



Sexual diff^erences are difficult to elucidate in the ciliates because the 

 picture is confused by two kinds of possible differences. The two con- 

 jugants may show differences in size, shape, or other characteristics. 

 These diff^erences between the two conjugants entering the union may 

 be interpreted as indicating maleness and femaleness. In a number of 

 forms the differences are slight but fairly constant, as in Miyashita's 

 (1927) "macroconjugants" and "microconjugants" oi Lada tanishi. In 

 the Vorticellidae, on the other hand, the difference in size and behavior of 

 the microconjugant and the macroconjugant is very striking, far greater, 

 in fact, than could be explained on the grounds of fluctuating variation. 

 The small free-swimming form that seeks out and fertilizes the large 

 sessile form could reasonably be called the male conjugant, and the large 

 form may be considered a female conjugant. In the Vorticellidae and in 

 Metopus sigmoides (Noland, 1927), mutual fertilization is not accom- 



