Their idea appears to me a sort of symbolism rather than a scientific attempt to explain sex. If their view had a real value, it ought to be possible to determine the sex of the developing organism with precision by regulating the conditions of its growth, and yet we cannot do this, nor do the authors make any claim of being able to do so. The hypothesis lacks the only support that can give it scientific standing, the proof of experiment. There have been made, from time to time, a number of attempts to show that the sex of the embryo is predetermined in the egg, and is not determined later by external circumstances. In recent years this view has come more to the front, despite the apparent experimental evidence which seemed in one or two cases to point to the opposite view. One of the most complete analyses of the question is that of Cuénot, who has attempted to show that the sex of the embryo is determined in the egg, before or at the time of fertilization. He has also examined critically the evidence that appeared to show that external conditions, acting on the embryo, may determine the sex, and has pointed out some possible sources of error that had been overlooked. The best-known case is that of the tadpole of the frog, but Cuénot shows not only that there are chances of error in this experiment as carried out, but also, by his own experiments and observations, that the facts themselves are not above suspicion. He points out that at the age at which some of the tadpoles were when the examination was made, it was not always possible to tell definitely the sex of the individual, and least of all by means of the size alone of the reproductive organs, as was supposed, in one case at least, to be sufficient. In his own experiments he did not find an excess of one sex over the other as a result of feeding. Cuénot points out that Brocadello found that the larger eggs laid by the silkworm give rise to from 88 to 95 per cent of females, and the small eggs to from 88 to 92 per cent of males. Joseph has confirmed this for _Ocneria dispar_, and Cuénot himself also reached this conclusion. Korschelt found that the large eggs of Dinophilus produced females and the small ones males. Cuénot experimented with three species of flies, and found that when the maggots were well nourished the number of the individuals of the two sexes was about equal, and when poorly nourished there were a few more females in two cases, and in another about the same number of males and females. It has been claimed that the condition of nourishment of the mother may determine the number of eggs of a particular sex, but Cuénot found, in three species of flies which he raised, that there was a slight response in the opposite direction. He concludes that the condition of the mother is not a factor in the determination of sex.