Sex Determination in Phylloxerans and Aphids 327 
sex in animals, since many points of resemblance appear in the 
two great divisions of the living world. 
Blakeslee recognizes two groups of moulds; viz: homothallic in 
which the thalli are all equivalent, conjugation taking place between 
parts of the same thallus or between any two thalli; and hetero- 
thallic in which the thalli are of two sorts, designated by the sym- 
bols (+) and (—). If spores are taken from the (+) or (-—) 
strain they produce a strain exactly like the parent. Phycomyces 
has been cultivated in this way through 107 generations, and 
Mucor mucedo through 106 generations without change in their 
sexual behavior. It appears that the (+) and the (—) strains 
perpetuate themselves indefinitely as such. If a (+) and a (-—) 
strain are brought together zygospores are produced by the union of 
equivalent parts from each thallus. If two (—) strains meet, or 
two (+) strains no sexual process occurs. From the zygospore 
a sporangium is formed containing many spores. It 1s here that 
the discoveries are extremely important, for the spores in the 
same sporangium are some male, others female. 
Another mould Sporodinia is homothallic, the mycelium (game- 
tophyte), the germ tube (sporophyte), and the sporangia are all 
alike. In other words from the zygospore a sporangium arises 
containing only one kind of spore which produces a homothallic 
mycelium. ‘There is no visible separation here into (+) and (—) 
elements, even although the conjugation phase of the mycelium be 
so interpreted. Such a form suggests the conditions found in an 
hermaphroditic animal, although the two kinds of germ cells in the 
latter present an external difference. 
A third type, Mucor mucedo, has separate (+) and (—) mycelia. 
By their union a zygospore results which gives rise to a sporangium 
containing in some cases only (—) spores (to judge from their 
mycelial product) and in other sporangia only (+) spores. Both 
sorts of spores do not appear inthe same sporangium despite the fact 
that the zygospore has been produced by the union of a (+) with 
a (—) strain. This remarkable discovery presents a quite differ- 
ent phase of the sex question. ‘The case is paralleled, as Blakeslee 
points out, among the flowering plants by such a form as the Lom- 
bardy poplar, where the fertilized egg produces in some cases a 
