THE PHYSIOLOGY OF FKR IIUZATION IN CILIATKS 289 



bur the marcs can srill be scpararcd wirhour injury. Subscqucnrly 

 each mare produces a paroral cone (Diller, 1936) more posreriorly 

 (Fig. lb). The cones of rhe mares overlap and finally fuse. Afrer 

 paroral cone fusion rhe mares cannor be separared. 



Afrer breakdown of rhe agglurinarcs rhe released animals will 

 nor give maring reacrions. Clearly rhey have undergone a physiologi- 

 cal change, a loss of maring reactiviry, Ar rhis rime (P. anrelia) rhe 

 firsr signs of nuclear acriviry appear. These involve a migrarion of 

 rhe micronuclei from rheir usual posirion in rhe viciniry of rhe macro- 

 nucleus and an enlargemenr of rhe micronuclei in prepararion for rhe 

 rirsr meioric prophase. The meioric divisions, rhe posr-meioric division 



Fig. 1. Types of union in conjugating and pseudo-selfing paramecia. 



(a) Holdfast union in early conjugants and pseudo-selfing animals, (b) Holdfast 

 and paroral cone union in more advanced conjugants. 



forming rhe rwo generically idenrical (Sonneborn, 1939a) pronu- 

 clei, macronuclear breakdo\\'n, and rhe passage of rhe migrarory pro- 

 nucleus ro rhe paroral region rhen follow. 



The holdfasr and paroral unions, loss of maring reacriviry, and 

 meioric and macronuclear evenrs all follow in a very precise and 

 orderly sequence in conjugarion. The obvious facrs rhar rhese changes 

 are induced in rhe conjugarion process and rhar rhey occur in a pre- 

 dicrable sequence suggesrs rhar rhese various changes are inrerrelared 

 and rhar rhey may be iniriared by a few or even a single reacrion in 

 conjugarion. In orher words, conjugaring paramecia are acrivared in 

 rhe same sense rhar rhe merazoan egg is acrivared by rhe ferrilizing 

 sperm. Indeed, some manifesrarions of acrivarion in rhe merazoan tgg 

 and in ParLWJeciinn are srrikingly analogous. 



Autogamy . Essenrially rhe same series of physiological and 



