304 SEX IN MICROORGANISAIS 



to the initiating mechanism. This follows from the fact that CM ani- 

 mals can activate normal animals. A second possibility requires that 

 each conjugant activate its mate by an independent system. Thus, an 

 inducing substance / in one animal combining with a reacting sub- 

 stance R in its mate might activate the latter. A reciprocal arrange- 

 ment of similar substances (R\ V) would be required to activate the 

 second animal. These relations are represented in Fig. 3 b, and it is ap- 

 parent that the CM block could reside in the activating mechanism if 

 the CM animals lacked R. Since there is no evidence that two such 

 independent systems operate in activation, the first alternative view 

 is accepted. Thus the CM block is placed internal to the initiating 

 mechanism in Fig, 2. 



Sonneborn (1942b,c) has described another "can't mate" stock 

 in F. aurelia. Animals of this type I (variety 1 ) stock gave good mat- 

 ing reactions with normal type II animals, but they failed to form 

 lasting pairs. Sonneborn describes pairs of type II animals from the 

 mixtures, but his account is not sufficiently detailed to determine if 

 these were pseudo selfing pairs. Sonneborn's (1942b) statement that 

 the clumps "continually break up and reform" suggests that loss of 

 mating reactivity was not induced in the type II animals, but this is 

 difficult to establish with certainty except by a controlled experiment 

 (Metz and Foley, 1949). Unfortunately neither the mutant type I 

 or the normal type II animals from such mixtures were examined 

 cytologically for nuclear evidence of activation. 



As Sonneborn (1949) points out, little information is available 

 regarding the relationship between holdfast substance formation, loss 

 of mating reactivity, paroral cone formation, meiosis, and macronu- 

 clear breakdown. These may arise independently from a main chain 

 as indicated in Fig. 2, or one or more of them may be sequential to 

 another. The available information regarding each of these will be 

 given in the order listed. 



Holdfast Substa?]ces. Holdfast union may result from interac- 

 tion of special holdfast substances which are produced as an early 

 manifestation of activation and which are not mating-type specific 

 (Metz and Foley, 1949). Such holdfast substance formation may 

 branch from the main activation chain between a and c (Fig. 2) since 

 autogamous animals never form holdfast or other unions. However, 

 the intimate association of animals obtained only in mating reaction 

 agglutinates may be necessary for holdfast union. The action of cer- 



