294 BIOLOGY OF THE PROTOZOA 



the macrogamete is smaller than the migrating mierogamete 

 (Fig. 144, p. 281). In the Vorticellidae the macrogamete remains 

 attached while the mierogamete is free-swimming. 



In hologamous fertilization by conjugation there is no universal 

 mode of fusion. In the majority of ciliates with adoral zones the 

 fusion area is usually the anterior region of the peristomial furrow, 

 the mouth as a rule being involved (e. g., Fig. 146). In exceptional 

 cases the mouth itself is involved in the protoplasmic bridge between 

 the two conjugants (Paramecium sp. Didinium nasutum, Spathidium 

 spathula). In Stentor fusion is lateral. Dogiel (1923, 1925), describes 

 an interesting case of conjugation in Cycloposthium bipalmatum. 

 Here the two individuals are united end to end, fusion occurring 

 at the borders of the peristomes, leaving the membranelles of the 

 adoral zone intact in a common conjugation cavity (Fig. 146). 

 The wandering pronuclei are provided with tails and, sperma- 

 tozoa-like, break through the anterior wall and into the conjuga- 

 tion cavity from which each enters the other conjugant by way of 

 the mouth. 



A. Meiotic Phenomena. The meiotic phenomena in many Pro- 

 tozoa are apparently started by stimuli resulting from contact and 

 partial fusion and may be divided into three types: (a) Conjugant 

 meiosis, or maturation processes occurring only after union of the 

 participating cells; (b) gametic meiosis (Wilson), or types in which 

 the maturation processes are antecedent to union; and (c) zygotic 

 meiosis (Wilson) characteristic of forms in which meiotic divisions 

 occur in the zygote subsequent to the fusion of the nuclei. The first 

 of these is illustrated by conjugating Infusoria; the second by the 

 great majority of types in which fertilization is accomplished by 

 permanent fusion of gametes; and the third by a few known cases 

 among the Sporozoa. 



(a) Conjugant Meiosis. — In mature ciliates the protoplasmic 

 organization is such that the stimulus received on contact is appar- 

 ently all that is needed to start up the nuclear activities associated 

 with the phenomena of chromosome reduction and preparation of 

 the pronuclei. These activities furthermore, have to do almost 

 entirely with the micronuclei. Macronuclei take no part in the 

 process of fertilization but are important in the subsequent reor- 

 ganization. 



With one or two exceptions (Trachelocerca phoenicopteriis, Spiro- 

 stomum ambiguum, etc.) all of the free-living ciliates thus far 

 described agree in the general course of their maturation phe- 

 nomena. Maupas (1889), the first to make a comparative study 

 of different ciliates during conjugation, described eight successive 

 phases of the process which are still applicable to practically all 

 ciliates. Of these, Phase A is characterized by the swelling and 

 early changes of the micronucleus; Phase B is the period of the 



