The question is often asked as to which is the time when the embryo is most sensitive to outward impressions, but as yet there is no sufficient body of evidence to show that at any particular time more than another (unless it be on the actual day of conception, see Chapter II) is the power of influence greater than any other. Is it possible to pre-arrange, to determine the sex of the child which is voluntarily conceived? Since earliest human experiences have been recorded, this has formed the theme of some writers and thinkers, and a variety of opinions have been expressed, theories propounded, and rules for the production of a girl or boy at will have been given. Each of the views, however, still remains far from being established, and damaging exceptions may be found to every theoretic rule. The impartial observer must feel that we are still unable to control the sex of the child. There are three main theories on this subject: (_a_) one is that the nature of the child which will be produced is already pre-determined in the ovum and sperm cell before they have united; (_b_) the second theory is that the critical moment which settles the sex of the future offspring is the moment of fertilization and the changes in the nucleus immediately resulting from it; (_c_) and the third theory is based on the view that the differentiation of the organs, which makes the difference in sex, take place at some stage in the embryo’s development after it is already a many-celled organism. The first named theory lies behind the advice which varies around the theme that according to whether the conception takes place from the egg cell grown in the right or the left ovary and testicle so will the child be a boy or a girl. Instances of the desired child proving to be of the sex “arranged for” by following out some such methods are of comparatively frequent occurrence, but to the scientist are completely counter-balanced by other and negative results. The second and third theories do not offer the same explicit application in practical advice. But all the practical advice, on whatever basis it is builded, appears to me to be laid on insecure foundations. In my opinion, the complexities of the factors which determine sex are such that it depends much less on the outward and visible nutrition of the mother, than on the inner and almost inscrutable quality of the nutrition of the ovum and spermatozoon before and immediately after fertilization has taken place.