288 PROCEEDINGS OF THE AMERICAN ACADEMY. 



without haviug suffered any ai^pareut change in their sexual character. 

 No esseutial ditferences have been observed between the (+) and (— ) 

 strains aside from that shown in their capacity for conjugation. 



At the temperature of the laboratory, zygospores do not form on the 

 artificial substrata usually employed as culture media. In the warm 

 oven, however, at a temperature of 26°-28° C, zygospores can be 

 obtained on potato agar the concentration of which has been increased 

 bv the addition of 4 per cent grape sugar, but on the same substratum 

 at the room temperature zygospores are not produced. In so far as ex- 

 periments have been tried, therefore, the optimum of temperature and of 

 concentration is higher for Mucor iv than for the other heterothallie 

 forms under cultivation. 



Mucor v. 



Zygospores of this heterothallie species were first found in a small 

 patch on a spontaneous dung culture in the laboratory, and by sei^aration 

 cultures and pure sporangial transfers it was eventually resolved, March 

 7, into its (+) and (— ) strains. Mucor v seems to be sexually the 

 most vigorous heterothallie form that the writer has under cultivation, 

 and no substrata have been tried on which it will not form its zygo- 

 spores in abundance when growths from the (+) and ( — ) strains are 

 apposed. 



In prepairing the culture jDhotographed (Plate IV, Figure 53). inocu- 

 lations of the two opposite strains were made at the jjlaces indicated by 

 the (+) and (— ) signs. The zygophoric hyphae seem strongly zy go- 

 tactic, and, recurving downward where the (+) and (— ) growths come 

 in contact, form a furrowed line of crowded zygospores. The zygo- 

 phores are of a more or less indefinite growth, and the zygosporic region 

 is soon extended on either side of the original areas of contact (Figures 

 53 and 57) in curved lines of light brown zygospores which ultimately 

 cover the whole surface of the culture as the two strains become com- 

 pletely intermingled. White lines are observable in the photographs 

 at the region of contact between the opposite strains. They are always 

 present shortly after the first zygospores have been formed, and consist 

 of zygospores in arrested stages of development. 



A detailed investigation of the process of conjugation in this sjjecies 

 has not been attempted, but essentially the same condition prevails as 

 has been already described in Macor Macedo. Plate III, Figures 

 40-44, are camera drawings taken at the intervals noted from a van 

 Tieghem cell culture which had been inoculated with a mixture of 



