518 BIOLOGY OF THE PROTOZOA 



In hologamoiis 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. 213). In exceptional 

 cases the mouth itself is involved in the protoplasmic bridge between 

 the two conjugants {Paramecium sp. Didiniumnasutum, Spathidium 

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

 describes an interesting case of conjugation in Cycloposthium. 

 hipalniatum. 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. 213). 

 The wandering pronuclei are j^rovided with tails and, spermatozoa- 

 like, break through the anterior wall and into the conjugation cavity 

 from which each enters the other conjugant by way of the mouth. 



A Meiotic Phenomena.— In relation to the union of cells in fer- 

 tilization the meiotic jjhenomena 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 majorit}' of types in which fertilization is 

 accomplished by permanent fusion of gametes ; and the third by a 

 few known cases among the Sporozoa and in the flagellate 

 ChJamydovionas. 



(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 

 witli 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 (Trachdoccrca phcenicoptenis , 

 Spirostovmm amhigwm, etc.) all of the free-living ciliates thus far 

 described agree in the general course of their maturation pheno- 

 mena. Maupas (1SS9) 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 

 first meiotic or maturation division; Phase C, the period of the 

 second meiotic division ; Phase D, the third nuclear division result- 

 ing in the formation of the pronuclei ; Phase E, the period of in- 

 terchange and union of pronuclei; Phase F, the period of the 



