240 THE LOWER FUNGI— PHYCOMYCETES 



honiothallic species. The difficulties of technique encountered 

 in following the sexual reactions in a tangle of filaments 

 were overcome by growing the homothallic species in a petri 

 dish between the ( + ) and (-) strains of the heterothalhc 

 species, and cutting channels in the agar between the different 

 thalH. The reacting filaments could then be studied in mid-air 

 in the channels. It was found that the sexual reaction occurs 

 between the larger female gametangium and the ( — ) strain 

 on the one hand, and between the smaller male gametangium 

 and the ( + ) strain on the other. Therefore the conclusion 

 is reached that the (+) strain of the heterothalhc species is 

 female and the ( - ) strain male. Subsequently, in his laboratory 

 a study of the biochemical differences between strains has 

 tended to substantiate this conclusion (Satina and Blakeslee, 

 1926 a, h; 1927). 



A good resume of the work on various aspects of sexuality 

 in the group was given by Blakeslee (1920 b). In later years 

 other investigators have somewhat broadened the field of our 

 knowledge in the Mucorales, and Blakeslee's prediction that 

 heterothalhc forms would be discovered in other groups of the 

 fungi has been fulfilled in the Saprolegniales, Peronosporales, 

 and other groups both of the lower and higher fungi. 



In several genera of the Mucorales species exists which are 

 parasitic on other members of the order. Burgeff (1924) has 

 studied several of these forms critically and advances the inter- 

 esting hypothesis that they have become parasitic as a result 

 of attempts at hybridization. He found that in the case of 

 Mucor (Parasitella) simplex the ( + ) strain parasitizes only the 

 ( — ) strain of the host Ahsidia glauca, while the ( — ) strain para- 

 sitizes only the ( + ) strain. In other cases the experimental 

 results are less conclusive, and Satina and Blakeslee (1926 6) 

 question the possibility of a wide application of the theory. 



The nuclear phenomena which accompany conjugation of the 

 gaemtangia and maturation of the zygospore are as yet imper- 

 fectly understood. The small size of the nuclei and the presence 

 of a large amount of oil in the zygospore render the problem 

 of staining a difficult one. A considerable number of forms 

 have been investigated, but the published accounts are con- 

 tradictory, incomplete, and unconvincing. All students of the 

 group have agreed that the progametangia and young game- 

 tangia are multinucleate, and several describe the disintegration 



