BLAKESLEE.— SEXUAL REPRODUCTION IN THE MUCORINEAE. 209 



It may be here stated that the writer has at present under cultivation 

 over a dozen forms, including both types, which will conjugate whenever 

 the proper conditions are furnished. Of the heterothallic group five 

 species of the genus Mucor are represented, including M. iMucedo, besides 

 Rhizopus nigricans, Phycomyces niiens, Absidia caemlea, and an unde- 

 scribed species of a new genus. Of the homothallic are Sporodinia 

 grandis, Zygorhynchus Moelleri, Dicranophora sp., and two undescribed 

 species of Mucor. The undetermined Mucors will be provisionally 

 indicated by Roman numerals, IMucors i and ii being homothallic, 

 Mucors HI to vr heterothallic, while Mucor N represents the unde- 

 scribed new genus. 



It is the purpose of the writer to give in Part I a brief historical 

 review of the theories that have hitherto been advanced in regard to the 

 causes influencing zygospore formation, followed by a citation as com- 

 plete as the writer has been able to make it, of all the species of which 

 this form of reproduction has been reported. In Part II the original 

 research on the subject of this paper will be arranged under the different 

 species investigated. 



In this connection it may be advisable to define certain terms, some of 

 which have been used variously by different authors, and to give a short 

 account of the morphology of conjugation. In Rhizopus or in Sporo- 

 dinia, for example, wherever two adjacent (+) and (— ) hyphae touch, 

 the stimulus of contact may cause outgrowths to form ; which, by re- 

 maining mutually adherent, and increasing in size, gradually push apart 

 the hyphae from which they have arisen. These outgrowths — the 

 gametes of some authors (e. g. Leger, '96) — may better be termed 

 progametes, since each becomes divided by a septum into a proximal 

 portion, the suspensor, and a terminal cell, the gamete proper, and it is 

 by the union of these gametes that the zygote is formed. This termin- 

 ology brings the process into accord with other plants and with animals. 

 Here, instead of dividing and developing into an embryo, the zygote 

 becomes transformed into a single-celled, thick-walled, resting zygospore. 

 It sometimes haj)pens that, owing to unfavorable conditions, the gametes 

 cut off fail to unite and develop independently into thick-walled azygo- 

 spores. The formation of these spores may be considered abnormal, 

 but in certain species where conjugation is not known to occur, so-called 

 azygospores have been found to be produced regularly in great abun- 

 dance. These reproductive bodies have been considered in connection 

 with the zygospores in the literature cited, but it is impossible to state at 

 the present time what their significance can be. 



VOL, XL. — 14 



