1 8 PHYSIOLOGICAL TRIGGERS 



specificity (see refs. lo, 62, 107, 130 for detailed accounts of this breeding 

 specificity). The process involves several types of union between mates and a 

 variety of physiological and cytological changes in them. 



The initial step in conjugation is an adhesion of potential mates upon random 

 contact. Under appropriate conditions this initial adhesion assumes mass 

 proportions, involves tens or even hundreds of paramecia and results in the 

 agglutinative mating reaction. This initial adhesion or mating reaction is not 

 mediated by diffusible, water-soluble sex substances present in the medium 

 (45, 60, 61, 105, 106) but appears to result from interaction of substances that 

 are attached to or built into the surfaces of the cilia. These substances, the 

 'mating type substances' (60) are presumably specific, complementary sub- 

 stances which interact in antigen-antibody-like fashion (61, 62). Subsequent 

 to the mating reaction, conjugants unite more intimately (fig. i), lose their 

 mating reactivity and undergo a very precise series of nuclear changes. These 

 include disintegration of the macronucleus and meiosis of the micronucleus or 



Fig. I. Types of union in conjugation. 

 a, Holdfast union. This is the secondary 

 union that follows the initial agglutinative 

 mating reaction, b, Holdfast and paroral 

 cone union of more advanced conjugants. 

 The gamete nuclei are exchanged across 

 the bridge formed by the tertiary or 

 paroral union. 



micronuclei, followed by the formation of gamete nuclei. After reciprocal 

 cross fertilization the mates separate and proceed to reconstitute the original 

 nuclear apparatus (see refs. 107, 130 for detailed accounts of these nuclear 

 changes). 



Clearly this elaborate and precise series of events is induced in some way 

 during sexual association, for it ordinarily fails to occur in the absence of con- 

 jugation. Furthermore, since culture fiuids and filtrates are not involved in 

 conjugation, the inducing or triggering mechanism would seem to depend upon 

 contact of potential mates. The essential feature of such contact would appear 

 to be an interaction of substances. This view received striking support from 

 the discovery that dead paramecia can induce conjugation changes in living 

 animals. Paramecia that have been killed and fixed by appropriate treatment 

 with drastic agents such as formalin or |)icric acid (58 60) retain their mating 

 reactivity. Such dead i)aramecia adhere to and under appropriate conditions 

 agglutinate massively with living animals. This striking reaction is highly 



