THE PUVSlOIXKiV Ol" II ,R 1 Il.l/A HON IN CILIA lES 307 



(Dillcr, 1949), and, in P. Ciilkinsi, type 11, activated by formalin-killed 

 animals, mciosis is frequently arrested in the prophase of the first 

 division in spite of the fact that the macronucleus subsequently 

 breaks down in normal fashion (Met/, and Foley, unpublished). Far 

 more spectacular deviations from the normal process have been de- 

 scribed by Nanney (1952) in centrifuged Tetrahyviena conjugants. 

 In summary form, the evidence presented in this section shows 

 that holdfast substance formation could arise independently from a 

 main activation chain at a point between b and c in Fig. 2. Loss of 

 mating activity certainly arises beyond c. It may arise before d. 

 Therefore paroral cone formation, macronuclear breakdown, and 

 meiosis could be sequential to holdfast substance formation and loss 

 of mating reactivity, but there is no evidence for sequential order or 

 dependence among these three phenomena. Clearly none of the other 

 activation phenomena is dependent upon meiosis or a micronucleus 

 since all occur in amicronucleate animals. 



Physical Basis for the Mating Reaction 



According to the views presented here the mating-type sub- 

 stances perform three major functions in fertilization: (1) They 

 effect the initial adhesion of potential conjugants. (2) They supply 

 the primary specificity in conjugation. (3) Their interaction triggers 

 the entire series of activation changes. 



In view of their primary role in fertilization it is now essential 

 to characterize the mating-type substances in more clear-cut physico- 

 chemical terms. Specifically, three items of information are needed, 

 namely, the location of the mating-type substances, their chemical 

 nature, and their manner of interaction. These are considered below. 



Location of the Matmg-Type S^ibstances. All workers agree 

 that the initial adhesion involves the cilia of animals of complementary 

 mating type. Jennings (1939a) and Tartar and Chen (1941) noted 

 that the surfaces of F. bursaria, when united in the mating reaction, 

 were separated by a space that approximated the length of one cilium. 

 These observations are readily confirmed by casual observation, but 

 unfortunately there are no detailed descriptions of the mating reac- 

 tion union in the literature. Therefore, Metz and Pitelka (unpub- 

 lished) undertook to examine this reaction in F. calk'msi with phase 

 contrast optics. In this form the cilia appeared to agglutinate tip to 



