298 SEX IN MICROORGANISMS 



to give the complete or "adult" spectrum of mating reactions. Such 

 clones are reported in variety I, a four-type variety. Here the adoles- 

 cent clones in question mate at first only with two of the four mating 

 types. Upon reaching maturity they mate with a third type, as well, 

 giving the full spectrum of reactions. As Jennings points out, it is of 

 considerable interest that such clones mate either with types A and B 

 or types C and D. The adolescent clones are therefore type CD or 

 AB, respectively. CD clones mature to C or D, and AB clones mature 

 to A or B. No BC or AD adolescent clones are reported. These obser- 

 vations fit the a-(3 concept on the assumption that the two systems of 

 substances do not appear simultaneously as the clones reach adoles- 

 cence. Thus the synthetic mechanism which produces the a system of 

 mating substances may appear at adolescence, w^hereas the mechanism 

 for i^i substance formation does not function until later (maturity). 

 During adolescence, then, the clones have one or the other of the a 

 substances (A or a) and will mate with the two mature types possess- 

 ing the complementary substance (a or A) indicated in Table III. 

 Upon reaching maturity, one or the other of the ^ substances (B or 

 b) is also produced and the final mating type is established. 



Only one series of intervarietal reactions has been reported in 

 P. bursaria (Jennings and Opitz, 1944). One of the two mating types 

 in variety IV (type R) cross reacts with four of the eight types 

 (types E, K, L, M) in variety II. Therefore four of the eight mating 

 types in variety II are similar. They may have one mating substance 

 in common which in each cross reaction combines with the same 

 complementary substance of variety IV, type R, animals. 



Thus in the irregular "adolescent" and intervarietal reactions 

 the results support the hypothesis to the extent that half of the mating 

 types in a multiple-type variety should have one mating substance in 

 common. Unfortunately, no other data pertaining to this problem 

 are available. 



Activation-Initiating A4echanism in Paramechnn. 



Until recently little was known regarding the activation-initiat- 

 ing mechanism of fertilization in Par ante cium. However, since the 

 systematic development of methods for partially isolating this system 

 (Metz, 1946, 1947) our understanding has increased to the point 

 where more, is now known regarding the activation-initiating mecha- 



