622 FERTILIZATION 



plished, because both pronuclei of one member — the microconjugant in 

 the Vorticeliidae — pass over with the cytoplasm into the other conjugant. 

 One pronucleus of the donor and one pronucleus of the recipient fuse, 

 to form the functional synkaryon. The other two pronuclei may or may 

 not fuse, but in either case they eventually disintegrate. In Metopus the 

 conjugants separate and the remnant of the donor dies; in the Vorti- 

 celiidae the microconjugant fuses completely with the macroconjugant. 

 These are obviously fertilization types intermediate between copulation 

 and conjugation. 



The other category of differences is that exhibited between the wander- 

 ing and the stationary pronuclei which are produced in the same con- 

 jugant. They are usually considered to be male and female pronuclei 

 respectively. Here the only apparent difference may be in their behavior, 

 as is the case in the majority of ciliates studied. In Euplotes patella there 

 is a slight difference in size between the wandering and the stationary 

 pronuclei, and there is a special zone of cytoplasm which accompanies 

 the wandering pronucleus in its migration. In Cycloposthium hipalma- 

 tum, however, Dogiel ( 1925 ) has described a spermatozoon-like wander- 

 ing pronucleus, which is in striking contrast to the rounded stationary 

 pronucleus. These illustrations may be considered as representing stages 

 in the evolution of distinct sexual differences between pronuclei of 

 ciliates. 



If we assume that differences between members of a conjugating pair 

 indicate sexual differentiation, then we would have male and female 

 individuals both producing structurally isogamous but functionally an- 

 isogamous pronuclei, as in Chilodonella (Chilodon) uncinatus (Enriques, 

 19O8; MacDougall, 1925). In other cases we would see male and 

 female conjugants both producing pronuclei which are functionally and 

 structurally differentiated as male and female, as in Cycloposthium (Do- 

 giel, 1925). 



If we consider the differences in behavior and structure between the 

 wandering and the stationary pronuclei as indicating sexual differences, 

 then we must consider the parent conjugants as hermaphrodites, and any 

 differences between conjugants would then be a leaning toward male- 

 ness or femaleness on the part of an hermaphroditic organism. Viewed 

 in this light, members of the Vorticeliidae have lost their double nature, 

 and the microconjugant has come to produce only male functional pro- 



