304 PROCEEDINGS OF THE AMERICAN ACADEMY. 



showed no further development than the production of progametes 

 which in some cases had distinguished th^ir gametes on one side. 

 Pure cultures were made of this form which showed such a peculiar 

 reaction, and labelled M. Mucedo vur. A. Subsequent contrasts on a 

 large variety of substances containing available nutriment in more or less 

 concentrations showed that this arrest in the development of zygospores 

 between var. A and the (+) strain of M. Mucedo was constant, and not 

 due, as was at first thought possible, merely to the inability of the progam- 

 etes, for some reason unusually abundant, to obtain from the substratum 

 sufficient nourishment for their further development. Var. A in tube 

 cultures is much slower and less luxuriant in growth, and is distinguished 

 at once from M. Mucedo by the light yellow coloration of sporangia, 

 hyjjhae, and mycelium. In addition there are certain slight structural 

 differences which made it seem improbable that the form could be other 

 than a distinct species, and suggested that the imperfect conjugations 

 observed with the (+) strain might be considered analogous to a process 

 of hybridization. 



The opposite strains of the nine heterothallic species under cultiva- 

 tion, which represent five different genera, were accordingly contrasted 

 with one another, and it was found that the opposite sti'aius of every 

 species are capable of hybridizing with strains of other forms. In this 

 way it has been possible to arrange the opposite strains of all the dif- 

 ferent species in two series according to their interaction with one 

 another. Mucor v has been found most active in the production of 

 hybrids, and has consequently been the form chosen as a standard with 

 which the other species have been tested. All strains that have been 

 found to hybridize with the strain of this species, which for reasons to 

 be shown presently has been called (+), have been placed together in 

 Table XII in the column on the left, while those that hybridize with 

 the strain called (— ) have been placed in the column on the right. 

 Continuous lines connect those strains between which hybridization has 

 been accomplished, and dotted lines connect those strains between which 

 attempts at hybridization have been unsuccessful. It will be noticed 

 that in no case has the position of a strain in either column been deter- 

 mined by positive hybridization in tests with less than two other species. 

 By an examination of the table it will be seen that in every instance 

 when a differentiation is apparent in the opposite strains, the strain show- 

 ing the less vegetative luxuriance has been located by the hybridization 

 tests in the column on the left. In view of their less luxuriant charac- 

 ter the strains in this column have been called (— •), while those in the 



