SEXUAL REPRODUCTION 81 



condition is essentially similar to that in dioecious plants 

 and animals, although morphological differentiation is not 

 so marked in the Mucorineae. The term heterothallic is 

 used to designate those forms that are dioecious, as in the 

 Rhizopus type; whereas homothallic is used for herma- 

 phrodite forms of the Sporodinia type. Hybrids have been 

 produced between -f- and strains of different species of 

 the heterothallic type. The following is the author's 

 summary of this most interesting discovery : 



(1) The production of zygospores in the Mucorineae is 

 conditioned primarily by the inherent nature of the 

 individual species and only secondarily by external factors. 



(2) According to their method of zygospore formation, 

 the Mucorineae may be divided into two main groups, 

 which have been termed respectively homothallic and 

 heterothallic. 



(3) In the homothallic group, comprising the minority 

 of the species, zygospores are developed from branches of 

 the same thallus or mycelium, and can be obtained from 

 the sowing of a single spore. 



(4) In the heterothallic group, comprising probably a 

 large majority of the species, zygospores are developed 

 from branches which necessarily belong to thalli or mycelia 

 diverse in character, and can never be obtained from the 

 sowing of a single spore. Every heterothallic species is 

 therefore an aggregate of two distinct strains, through the 

 interaction of which zygospore production is brought about. 



(5) These sexual strains in an individual species show 

 in general a more or less marked differentiation in vege- 

 tative luxuriance, and the more and less luxuriant may be 

 appropriately designated by the use of + and signs 

 respectively. 



F 



