3. On the basis of all these experiments and theories it is comparatively easy to explain a number of phenomena concerning sex ratios which before had been very puzzling. In bees it had been shown many years ago by Dzierzon that the males develop from unfertilized eggs while the females, queens and workers, develop from fertilized eggs. This is intelligible on the assumption that the unfertilized egg contains only one X chromosome while the spermatozoön carries into the egg the second X chromosome. But if the male bee produces two types of spermatozoa we should expect that only one-half of the fertilized eggs should be females, the other half males. But it happens that of the two types of spermatozoa only one is formed since in one of the cell divisions which lead to the formation of spermatozoa one viable spermatozoön only is formed while the other one perishes. It is, therefore, quite possible that it is the female-producing spermatozoön which survives while the male-producing spermatozoön dies. It is occasionally observed that an insect shows one sex on one side of its body and the opposite sex on the other side. Boveri suggested that this phenomenon of gynandromorphism is due to the fact that the spermatozoön for some unknown reason does not fuse with the egg nucleus until after the egg has undergone its first cell division. In this case it fuses with the nucleus of one of the two cells into which the egg divides (or in some cases even one of the later cells?). As a consequence the one-half of the embryo which arises from the cell which was not fertilized would have only one X chromosome and in a case like the bee would develop parthenogenetically, while the other half of the body, developing from the cell into which a spermatozoön has penetrated, would be fertilized. The latter half of the body would be female, the former male. In his last paper before his untimely death, Boveri has given proof for the correctness of this interpretation as far as gynandromorphism in the bee is concerned.[187] [187] Boveri, Th., _Arch. f. Entwcklngsmech._, 1915, xlii., 264. It seems to be generally true that where sexual reproduction leads only to the formation of females the case finds its explanation in the fact that the male-producing spermatozoa perish and only the female-producing spermatozoa survive. Such an observation was made by Morgan on a certain species of phylloxerans. The slight preponderance in the number of one sex which is occasionally found--an excess of six per cent. males over females in the human race--may well find its explanation on the assumption of a slightly greater mortality of the female-determining spermatozoa.