Sex Determination in Phylloxerans and Aphids 331 
We might conclude on the other hand that the egg always “‘retains”’ 
one sex factor and the polar body the other (while there are two 
kinds of functional sperm); the egg would then be homozygous 
but if two kinds of sperm are produced one must be functionless 
unless like can fertilize like. The third possibility shown by the 
moulds does not seem to appear in these dioecious mosses; viz: 
where one sporophyte produces only male spores and another 
female spores. Moreover in mosses all sporophytes are alike, 
while in animals and in some dioecious plants two types of sporo- 
phytes exist, males and females. If in these all the sperm are 
males (+) and all the eggs female (—) direct comparison with the 
mosses cannot be made. For the sake of clearness it may be 
well to keep these contrasts in mind. 
The chromosomal cycle in ferns shows apparently that there is 
no necessary connection between the reduced number of chromo- 
somes and the segregation of sex.18 Eachspore produces a prothal- 
lium—homologous with the protonema of mosses—with the 
reduced number of chromosomes in all of the cells. Such a pro- 
thallium may produce both archegonia and antheridia, the former 
containing each an ovum, the latter spermatozodids. ‘This fact 
shows that the gametes of one or the other sex are produced long 
after the reduction has occurred, and on one and the same plant. 
If it can be shown that the spermatozodids of a prothallium can 
fertilize the egg-cell of the same prothallium, then they must be of 
opposite signs. The fact that in certain ferns it has been possible 
to suppress the development of the archegonia or of the antheridia 
by changing the external conditions—a result that Hertwig and 
Whitney have also obtained in Hydra—obviously ‘does not show 
that the sex has been changed, but that in a hermaphrodite form 
certain conditions are more favorable for the production of one or 
the other gamete. 
Previous to the recent agitation concerning the sex of the gametes 
and their reciprocal fertilization, it was generally taken for granted 
by zodlogists and botanists that any sperm may fertilize any egg. 
18 In hermaphroditic prothallia the reduced group of chromosomes might be supposed to contain both 
elements that are separated only at the time of formation of the reproductive organs or of their germ 
cells. 
