212 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



species comes in contact with the (— ) of the other, a white line will be 

 observed which a microscopic examination has shown is composed of 

 incipient stages of conjugation wherein abundant progametes, and less 

 frequently gametes, have been formed. It is an important fact that only 

 when strains having unlike signs come in contact do these attempts at 

 hybridization occur. 



The accompanying diagram will perhaps render clearer the condition 

 shown in the culture photographed. The continuous line between 



Mv(+) and Mv(— ) indicates the zygo- 

 spores formed by the two sexual strains 

 of this species. The broken lines 

 between Mv(+) and Mn(— ), and 

 between Mv(— ) and Mn(+), indicate 

 the lines formed by the attempts at 

 hybridization between those strains of 

 the two species which correspond to 

 different signs. The dotted lines in- 

 dicate the contacts of those strains of 

 the two species corresponding to like 

 signs. No distinct line is observed 

 here in the photograph, and a mi- 

 croscopic examination shows that no attempts at hybridization have 

 occurred. 



By making contrasts between the (+) and (— ) strains of those 

 heterothallic species which have been producing zygospores in the 

 laboratory it has been possible to arrange in a series all strains which 

 had been marked (+) on account of their vegetative characters, and in 

 another series all those marked (— ) on the same basis. Those in the 

 (+) series are found to be capable of forming imperfect hybrids with 

 those of the ( — ) series, but no attempt at conjugation can ever be ob- 

 tained between strains with like signs, either in the same or in different 

 species. 



It is further possible to place in the proper sexual columns the strains 

 of those species (e. g. Rhizopus) in which a vegetative differentiation has 

 not yet been determined (cf. table, p. 305), by means of the tendency 

 already referred to, which leads to attempts at hybridization when (+) 

 and (— ) strains of different species, and even genera, are made to grow 

 in contact with one another. Such an attempt at hybridization between 

 Absidia and Rhizopus, for example (Plate I, Figure 18), shows in the most 

 advanced cases the characteristic appendages from the Absidia suspensor, 



