328 J. H. Morgan 
male tree, i. e., one producing only pollen, in other cases a female 
tree, i. €., one producing only ovaries. On this view any pollen 
erain could fertilize any egg, for all the eggs are of one sign and 
all the sperm of the opposite. 
What determines whether the fertilized product gives rise in one 
case to a female and in another to a male is quite unknown. The 
three types of sexual reproduction represented by these three 
moulds, Phycomycetes, Sporodinia and Mucor, illustrate three 
important phases of sexual reproduction; the first shows that male 
and female strains develop in the same sporangium; the second 
shows that no dissociation occurs in the sporangium; and the third 
that one sporangium contains spores only of one sign and another of 
the other sign. Either the first or the third types might be made the 
basis for comparison with unisexual animals; if the first, there 
would be (—) and (+) sperm, (—) and (+) eggs, with reciprocal 
fertilization; if the third, all the sperm would be of one sign, all the 
eggs of the other sign; any spermcould fertilize any egg. Unfortu- 
nately we lack evidence for animals that show to which type they 
belong. Both alternatives will be considered later. It will also 
be noted that the ( +) and ( —) signs are used with respect to fertili- 
zation, not with respect to the kinds of ‘“‘sporophyte” that arise from 
the zygospore. Externally these sporophytes in moulds are alike 
whatever their physiological possibilities. Sex is used in animals 
and plants with regard to the kind of sporophyte that results from 
fertilization; these are of two kinds, male or female. If the third 
type of reproduction represents the transition to the higher type, 
two facts of much importance will be noticed; first that the male 
eametes all have one conjugation sign, and the female gametes the 
other. Hence every sperm can fertilize every egg. Sex therefore 
relates to the character of the sporophyte, rather than to the sign 
of the gametes; yet one sign is associated with one sex and the other 
with the opposite. The second fact 1s this; since opposite signs 
must unite the product is hybrid (+ and —) yet only one sign 
comes to development in the male, and the other in the female. 
Whatever determines the sex, therefore, must also determine the 
sign of its gametes. Uf the former is a quantitative relation, the 
latter must be also. 
