534 



THE AMERICAN NATURALIST [Vol. XLYIT 



set or litter, whether the number of individuals in a set 

 be 2, 3, 4 or 5, are of the same sex. Of these 182 sets 88 

 were female and 94 male, which would seem to indicate 

 that the two sexes are about equal in numbers. A total 

 of 210 sets have come under the writer's observation, and 

 of these four showed 5 fetuses, four showed 3 normal and 

 1 degenerate individuals, and in one case twins were 

 born, due probably to the degeneration of a pair of 

 fetuses. There are no authentic cases of less than four 

 embryos being produced, but there are four cases, or less 

 than 3 per cent, of the total number, in which there is 

 exhibited a tendency toward an increase in the normal or 

 typical quadruplet condition. This may be a progressive 

 tendency and might conceivably result in numbers of 

 fetuses resembling those produced by the mulita. The 

 fact that the individuals of a polyembryonic litter are 

 invariably of the same sex supports certain current views 

 regarding the problem of sex determination. In partic- 

 ular it shows clearly that sex must be determined prior to 

 the separation of the embryonic materials from which 

 the four fetuses arise. Since, from the standpoint of 

 cell lineage, this separation must take place at least as 

 early as the cleavage stages, it would appear practically 

 certain that sex is predetermined in the undivided 

 oosperm. It has been claimed that the data with refer- 

 ence to sex in the armadillo might as readily be used as 

 evidence of the control of sex by environment; for it is 

 claimed that the environment of the four fetuses in \ 

 common chorion is as nearly identical as it could be made 

 under controlled conditions'. I claim, however, that there 

 is no greater environmental uniformity here than exists 

 in cases where several fetuses develop simultaneously in 

 a single uterus. In both cases the individual fetuses have 

 separateamnia, separateplacentreand unmixed fetal blood. 

 The enclosure within a common chorion is a matter of sec- 

 ondary importance since each fetus is isolated completely 

 in the really important ways just mentioned. Moreover it 

 is certain that pronounced differences in nutrition and 



