308 PROCEEDINGS OF THE AMERICAN ACADEMY. 



var. A. "Whether a similar relation exists in other hybrids between 

 the strains characterized bj the greater development has not been deter- 

 mined. Var A has been grown in pure cultures and continued to the 

 twentieth generation without suffering any apparent change in its growth 

 or in its sexual activity. 



Rhizopus and Absidia were the first species after Mucor Mucedo and 

 var. A between which hybrids were obtained. On flour paste, which is 

 a suitable substratum for the production of zygospores of Rhizopus but 

 less favorable for those of Absidia, hybrids were meagre in development, 

 and could be found only between Absidia (+) and Rhizopus (— ). On 

 potato agar, however, which is more favorable for the zygospores of Ab- 

 sidia but less favorable for those of Rhizopus, hybridization was more 

 active. The commonest condition in the process of this hybridization is 

 represented by Figures 16 and 17, Plate I. The hyphae belonging to 

 Absidia can readily be distinguished from those of Rhizopus by the blue 

 coloration of the former, but the difference in nature of the two pro- 

 gametes is strikingly apparent in the more advanced stages, where, as 

 is shown in Figure 18, the characteristic outgrowths have arisen from the 

 Absidia suspensor. In the instance figured a gamete has been developed 

 on the Absidia side and septa have formed in either suspensor. The 

 liroduction of gametes on both sides, however, is rare. 



That the sexual character in Rhizopus is not confined to special zygo- 

 phores is confirmed by the fact that when the (+) strain, for example, of 

 this species is contrasted with the (— ) strain of Mucor v, the stolons of 

 the former are thickly beset with hybrids which have arisen in response 



to the contact between hyphae of the 



^ -^^ sexually opposite strains. The hyphae 



y^ \ N. of the Mucor recurve from all sides 



/ i \ ^nd apply themselves to the stolons of 



/ f Mivit) I \ Rhizopus, presenting in cultures an 



\ \ \ appearance of yellow festoons radiat- 



„.......„„..„„,...; M y (+) ing from the line of contact. 



' I Mucor IV does not produce its zygo- 



te '^^"' , / spores at the room temperature of the 



1 / laboratory on the nutrients ordinarily 



' y^ used for the cultivation of other forms. 



^4..,___^ ^^^_,,^ The accompanying diagram represents 



the condition in a stender dish culture 

 of potato gelatine. Mucor iv (+) and Mucor iv (— ) are contrasted 

 with each other in the centre of the dish, and on the sides are contrasted 



