270 PROCEEDINGS OP THE AMERICAN ACADEMY. 



It has uow been shown that inequality in the size of the gametes and 

 suspeiisors is of no sexual significance, and a microscopic examination of 

 both strains shows no appreciable difference in the size and markings 

 of the spores nor in any other of the characters observed. Moreover the 

 gross appearance of their growth in test-tubes and in gross cultures will 

 not enable one to distinguish the strains, and experiments have not been 

 carried on as yet to determine if they possess any secondary physiological 

 characters, in addition to their primary sexual differences. 



When the strains of culture A were first separated it was thought that 

 the strain uow marked (+) on account of its hybridizing action with 

 other species was the one which showed a greater vigor of growth. In 

 a single bread culture where the (+) and ( — ) strains were opposed in 

 two parallel lines and zygospores formed at the contact of the opposite 

 growths, it was found that the (+) strain had grown up against the 

 cover, while the ( — ) strain showed a growth distinctly lower. It seemed 

 possible to explain the exclusively (+) character of all the sporangial 

 transfers taken in separating the strains of culture A by supposing that 

 when sown together there was a constant separation of sporangial growth, 

 the (+) sporangia being in the upper part of the culture where they 

 would be more readily taken for transfers. Further to test this suppo- 

 sition, pure transfers were made from five different sporangia taken from 

 different parts of the upper growth of a bread culture where the spore 

 material used in the inoculation of the two strains had been mixed in 

 approximately equal amounts. All of these transfers, when tested, turned 

 out to be (+) in agreement with the supposition mentioned. 



The difficulties which were encountered in obtaining other than the 

 (-}-) strain in separating cultures A and C (p. 255) pointed to a similar 

 conclusion, as well as the fact that it was definitely shown by a number 

 of tests of spontaneous infections of Rhizopus that (+) spores were the 

 more abundant in the air of the laboratory. Out of nine such infec- 

 tions tested the first five were (+), one was (— ), and three belonged 

 apparently to the group of neutral strains previously mentioned (p. 263). 

 Nevertheless, it should be mentioned that in a Petri dish culture which 

 had been accidentally infected from the air of the laboratory it was 

 found that two mycelial colonies had thus originated at opposite edges of 

 the dish and showed that they belonged to different strains by forming 

 zygospores when they came in contact. 



In marked contrast to the condition above described, culture H 

 (p. 259), in which the ( — ) transfers predominated in the ratio of 

 nineteen to one, seemed to indicate here that the relative conditions 



