FERTILIZATION 591 



gametes; so, if the size difference means a step in the direction of male- 

 ness and femaleness, then the colony is monoecious or hermaphroditic. 

 This obviously leads to the conclusion that, in the evolution of sex in 

 these forms, gametes became differentiated into male and female types 

 before the parent organisms did. The fact that a larger percentage of 

 primitive Metazoa exhibit hermaphroditism than do the higher forms 

 lends weight to the assumption that this is a general truth. 



In Eudorina elegans differentiation of gametes has become very 

 marked, and in Volvox the series is climaxed by such extreme differentia- 

 tion between the microgametes and the macrogametes as is seen in meta- 

 zoan sperms and eggs. Furthermore, the vegetative cells of the Volvox 

 colony are comparable to the somatic cells of the Metazoa, while rela- 

 tively few cells of the colony carry on the germ line. Another advance 

 seen in Volvox is that some species have developed the dioecious 

 condition, wherein some colonies produce only microgametes and others 

 produce only macrogametes. 



Among the Infusoria are found a few cases in which gametes are 

 formed that unite in complete and permanent fusion. This process is 

 therefore copulation, rather than the usual ciliate conjugation (see p. 

 617). In most cases of copulation in ciliates, the gamonts undergo re- 

 peated divisions, which result in the production of numerous small 

 "microgametes" which copulate with each other. Thus copulation of 

 gametes has been described for Trachelocerca phoenkopterus (Lebe- 

 dew, 1909), in the Opalinidae (Neresheimer, 1907; Metcalf, 1923), 

 and in Glaucoma (Dallas/a) jrontata (Calkins and Bowling, 1929). 

 While these gametes may differ a little in size and be called "micro- 

 gametes" and "macrogametes" by some, the differences do not appear 

 to be very significant. In general, they bear considerable resemblance 

 to the trophic individuals except in size. In the Vorticellidae, however, 

 gametes are formed which are truly anisogamous and which fuse per- 

 manently, although cytologically they more nearly resemble anisogamous 

 conjugants (see p. 621 below). 



In the ciliate Metopus stgmoides, Noland (1927) described a sexual 

 process which is somewhat intermediate between copulation and conju- 

 gation. Conjugants come together and join anteriorly, but instead of 

 exchanging pronuclei as conjugants usually do, most of the cytoplasm 

 and both pronuclei of one member of the pair pass over into the body 



