THE PHYSIOLOGY OF- FERTILIZATION IN CILIATES 299 



nisni in raravicciuiii than in any other organism. Partial isolation of 

 the activating system is achieved by killing and fixing paramecia 

 without destroying their ability to activate living animals. Such dead 

 paramecia may be regarded as a collection of highly specific sub- 

 stances adsorbed to or built into an inert carrier, the bulk of the dead 

 animal. By utilizing this partially isolated, static system, the activation- 

 initiating mechanism has been examined most thoroughly in P. mirelia 

 (Aletz, 1946, 1947, 1948; Metz and Foley, 1949) and somewhat less 

 extensively in P. calk'msi (Metz, 1948; Aletz and Butterfield, 1951) 

 and F. cmidatmn (Hiwatashi, 1949b, 1950). 



A\Mien properly prepared dead paramecia are mixed with reactive 

 li\ing animals of complementary mating type, the living and dead 

 animals promptly adhere and under favorable conditions form large 

 mating reaction agglutinates as in normal conjugation. These mating 

 reaction agglutinates break down after 1 to 2 hours {P. mirelia)^ re- 

 leasing the living animals. These may be freed as single individuals or 

 as "pseudo selfing" pairs (Fig. la) joined only at the holdfast region* 

 {P. aurelia and P. calkinsi; Metz, 1947, 1948). The released animals 

 (singles as well as pairs) then proceed to undergo meiosis and macro- 

 nuclear breakdown in normal fashion and according to the time 

 schedule of normal conjugants (Metz, 1947). None of these events 

 is observed in mixtures of living and dead paramecia of the same mat- 

 ing type. This activation of living by dead animals of opposite type 

 has been reported in types 7 and 8, variety IV of P. aurelia (Metz, 

 1947); type II of P. calkinsi (Metz, 1948); and types 2, 4, 5, and 8 of 

 P. cazidatum (Hiwatashi, 1949b). With this brief outline of the newer 

 methods we may now attempt to analyze the activation-initiating 

 mechanism in Parameciiivi. 



Activation through sexual processes in Paramecium requires con- 

 tact of potential mates. This follows from the facts that culture fluids, 

 filtrates, and the like have no specific efl^ect upon paramecia of op- 

 posite mating type (Sonneborn, 1937, 1939b; Kimball 1943; Metz, 

 1947); that animals which have been killed, fixed, and repeatedly 

 washed retain their ability to activate living animals; and finally that 

 the ability of dead (or living animals) to activate is directly related to 

 their ability to give mating reactions. Since the process of activation 

 in conjugation requires contact or union of potential conjugants, the 



* Hiwatashi (1951a) reports "pseudo selfing" pairs in P. caudatmn which 

 are united at both the holdfast and the paroral regions. 



