208 PROCEEDINGS OF THE AMERICAN ACADEMY. 



was impossible to continue the formation of zygospores. In the attempt 

 to grow a form of the Rhizopus type on nutrient agar in a more convenient 

 condition for sectioning, a separation culture was made from the midst 

 of the zygosporic region, and zygospores were found (May 2, '03) to 

 appear at the junction of certain of the mycelial colonies. By following 

 up this suggestion it was demonstrated that this species consists of two 

 strains or races which, when grown apart, produce only sporangia, but 

 which form zygospores when hyphae from the physiologically different 

 mycelia are allowed to come in contact. If we designate one strain by 

 the sign (+) we can conveniently use the (— ) sign to represent the other 

 without committing ourselves as to the sexual relation which the strains 

 may bear to each other. A (+) strain, for example, will never produce 

 zygospores on any medium if sown alone, nor if sown with some other 

 (+) strain however different in origin ; but if the complementary (— ) 

 strain be sown at the same time either mixed with the (+) or so disposed 

 that their mycelia can come in contact, zygospores are formed by the 

 union of gametes produced from the (+) and (— ) mycelia respectively. 

 The condition is essentially similar to that in dioecious plants and animals, 

 and although in this case the two complementary individuals which are 

 needed for sexual reproduction are not in general so conspicuously differ- 

 entiated morphologically as in the higher forms, such a morphological 

 difference is often distinctly visible, and, as hereafter noted, would un- 

 doubtedly be considered by systematists generally as an amply sufficient 

 basis for their specific separation. Inasmuch, however, as conjugation is 

 possible only through the interaction of two differing thalli, we can express 

 this fact by calling all species the sexual relations of which correspond to 

 the Rhizopus type, heterothalUc. In marked contrast to the conditions 

 just described, Sporodinia and the other members of the group of which 

 it is the type, invariably reproduce sexually under suitable conditions 

 when grown from a single spore. The zygospores thus originate from a 

 sino-le mycelium, and are comparable to hermaphrodites among the higher 

 plants. Such forms may tlierefore be called homotlidlic. 



In several of the heterothalUc forms experimented with, certain races 

 have been found which apparently cannot be induced to respond to the 

 (+) and (— ) strains, the existence of which has been demonstrated in 

 such species. Moreover it has been shown that in 3Iucor Mucedo the 

 power of conjugating possessed by the two sexual strains may be at will 

 entirely inhibited by cultivation under unfavorable conditions. These 

 " neutral " strains will be further discussed in connection with the indi- 

 vidual forms considered in Part II. 



