BLAKESLEE. — SEXUAL REPRODUCTION IN THE MUCORINEAE. 313 



(1) The production of zygospores in the Mucorineae is conditioned 

 primarily by the inherent nature of the individual species and only sec- 

 ondarily by external factors. 



(2) According to their method of zygospore formation, the Mucor- 

 ineae 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 there- 

 fore 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 vegetative luxuriance, and the 

 more and less luxuriant may be appropriately designated by the use of 

 (+) and (— ) signs respectively. 



(6) In heterothallic species, strains have been found which from their 

 failure to react with (+) and (— ) strains of the same form have been 

 called "neutral," and a similar neutrality may be induced by cultivation 

 under adverse conditions. 



(7) In all species of both groups in which the process of conjugation 

 has been carefully followed the swollen portions (progametes) from 

 which the gametes are cut off do not grow toward each other as 

 currently believed, but arise from the stimulus of contact between more 

 or less differentiated hyphae (zygophores), and are from the outset 

 always normally adherent. 



(8) In some species the zygophores have been demonstrated to be 

 mutually attractive (zygotactic). 



(9) In the heterogamic subdivision of the homothallic group a distinct 

 and constant differentiation exists between the zygophoric hyphae and 

 the gametes derived from them, but in the remaining homothallic forms 

 and in all heterothallic forms no such differentiation is apparent. 



(10) A process of imperfect hybridization will occur between unlike 

 strains of different heterothallic species in the same or even in different 

 genera, or between a homothallic form and both strains of a heterothallic 

 species. 



(11) By taking advantage of this character it has been possible to 



