THE PHYSIOLOGY OF FERTILIZATION IN CILIATES 3 1 7 



ferential action on the two mating types of variety 4, P. aurelia 

 (Metz, 1947). 



A differential action of nitrous acid was found in P. calkinsi 

 (Table \^). Type 1 animals mate after treatment with nitrous acid 

 at 0*^0, whereas type II animals fail to do so. The mating reactivity 

 of both is destroyed by nitrous acid at 25 °C. It is tempting to ascribe 

 the differential action of formalin and nitrous acid to inactivation of 

 amino groups. However, the action of both these agents is complex 

 (see French and Edsall, 1945; Olcott and Fraenkel-Conrat, 1947), 

 and no such conclusion is warranted until this very interesting rela- 

 tionship has been subjected to a more thorough biochemical analysis. 



From the chemical studies reviewed in this section it is evident 

 that mating-type-substance activity is dependent upon protein integ- 

 rity. Among amino acid residues sulfhydryl and disulfide groups are 

 not essential, but free alpha amino groups or terminal groups of basic 

 amino acids, phenolic groups, and perhaps other amino acid residues 

 appear to be essential. 



Ivnmmological Studies on the Mating Reaction. Since the 

 mating-type substances of Farainecium cannot be extracted, an immu- 

 nological analysis of these substances in sitn might be expected to 

 yield valuable information regarding their nature. In some experi- 

 ments antisera affect complementary mating types differentially 

 (Bernheimer and Harrison, 1941; Hiwatashi, 1951b); in others the 

 two types behave alike (Bernheimer and Harrison, 1941; Sonneborn, 

 personal communication). Such variable results are not surprising 

 since the experiments were based on the immobilization of living 

 animals, and the presence of a given immobilizing antigen is known 

 to depend upon both the genotype and the environment (Sonneborn, 

 1950; Beale, 1951), at least in P. aurelia. In view of the absence of 

 mating-type specificity in these studies and the ambiguity of the 

 immobilization test in this regard, Metz and Fusco (1948) designed 

 a direct test for anti-mating substance antibodies in immune sera. 

 P. aurelia was used in the original study, and the results have been 

 confirmed on a less extensive scale with P. calkinsi. Reactive formalin- 

 killed animals were treated with antiserum for a substantial period 

 (30 minutes to 2 hours) and then washed free of the excess antiserum 

 and tested for mating reactivity with living reactive animals of the 

 same and complementary types. In some tests the antiserum-treated 

 animals failed to mate. This clearly indicates that the mating-type 



