214 PROCEEDINGS OF THE AMERICAN ACADEMY. 



and its zygospores have not been seen simply because it has not been 

 found in contact with its complementary strain. On this basis we can 

 add to the heterothallic forms already known representatives of the genera 

 Syncephalastrum, Circinella and Chaetocladium, and this will extend 

 heterothallism to all the families of the Mucorineae recognized by 

 A. Fischer ('92) except the INIortierellaceae. No attempt has yet been 

 made to determine the sexual condition in this family. 



From the fact that the writer has demonstrated the possibility of 

 running out the sexual activity in Mucor Mucedo, and the apparent 

 impossibility of obtaining zygospores from contrasts made between 

 certain races of Rhizopus nigricans and (+) and (— ) strains of this same 

 species, it is further evident that neutral sti'ains, or at least strains in 

 which the sexual activity is dormant, exist in nature. The character of 

 the substratum, and also the nature of the forms contrasted, are important 

 factors to be considered in hybridizing. Phycomyces, for example, will 

 hybridize with Mucor Mucedo only on concentrated nutrients, but will 

 not hybridize with certain other forms under the same conditions. Con- 

 sequently a failure to obtain imperfect hybrids by contrasting a given 

 culture with the opposite strains of a test form proves nothing positive 

 in regard to the thallic condition of the culture under investigation. 



HISTORICAL REVIEW OF THE THEORIES RELATING TO ZYGO- 

 SPORIC REPRODUCTION IN THE MUCORINEAE. 



With the foregoing brief review of the more important matters treated 

 in the present research, and before considering them in detail, it has 

 seemed desirable to present a short summary of the more prominent 

 views which have been held by previous investigators regarding the 

 conditions influencing the formation of zygospores in the Mucorineae, 

 and the nature of the processes involved, to which is appended as com- 

 plete a statement of the reported occurrence of these bodies as it has been 

 possible for the writer to compile. 



In Ehrenberg's ( ['20]'29) work on Syzygites, which contains the 

 first mention of the occurrence of zygospores among the Mucorineae, he 

 recognizes the conjugation as comparable with the act in such algal forms 

 as Spirogyra and as of a probable sexual nature. Subsequent investi- 

 gators, however, who have advanced theories to account for the appear- 

 ance of zygospores, either deny that there is any sexuality in the process 

 of their formation, or consider such a possibility as without bearing on 

 the problem. Such explanations as have been offered are thus based in 

 general on the assumption that zygospore formation is primarily depend- 



