V. H. Blackman. 
216 
Blakeslee confirmed this result by numerous observations of 
Mucor Mucedo, and lias extended bis observations to the Mucorineae 
generally 1 . He finds that the majority of the members of the group 
examined belong to the type of Mucedo Mucedo , and these he classes 
as heterothallic forms, since two different thalli are required for 
zygospore production. In these forms any given thallus bears 
spores which give origin to thalli of the same nature as the parent 
thallus. 
In a minority of forms ( Sporodinia grandis, Spinellus fusiger, 
some species of Mucor, Zygorhynchus Moelleri, Dicranopliora sp.) 
the zygospores were found to arise by the interaction of hyphae of 
the same mycelium, and such forms are termed homothallic since 
they possess only one kind of mycelium. 
Blakeslee finds that the two different strains of the heterothallic 
forms can generally be distinguished by a slight difference in the 
luxuriance of their growth, although they are morphologically 
indistinguishable ; but even this difference is not always present. 
Owing to this usual distinction the terms (+) and (—) were 
suggested for the two strains. The further very interesting obser¬ 
vations were made that imperfect hybridization—the production 
of the early stages of zygospores which, however, never developed 
any further—would take place, between strains of different 
heterothallic forms, and between both ( + ) and (—) strains of 
heterothallic forms on the one hand, and homothallic forms on the 
other. A homothallic form sown between the two strains of a 
heterothallic form would thus give rise to two lines of imperfect 
zygospores. Owing to this power of hybridization between the 
various forms of the Mucorineae, it is possible to place in their proper 
category of ( + ) or (—) the strains of such a heterothallic form 
as Mucor Mucedo, which shows no visible difference in the luxuriance 
of growth of the two strains. By the same means also the ( 4 -) or 
(—) nature of the thalli can be tested in those heterothallic forms 
of which only one kind of strain has hitherto been observed. 
These results are of extraordinary interest and throw a flood 
of light on the conditions of zygospore production in the group ; but 
the process of development of the zygospores of the heterothallic 
forms can hardly be on the same plane with ordinary sexual 
processes, as Blakeslee appears to assume. The discussion of 
questions of sexual reproduction is unfortunately rendered difficult 
1 A. F. Blakeslee. Sexual Reproduction in the Mucorineae. Proc. 
Amer. Acad. Arts and Sciences XI., 1904, p. 205. 
