Mayrhofer was led by critical notices and the observations of others, and, further, by his own experiences, to conclusions which he set forth in propositions, some of which I select here.
In the plants and the lower animals food plays a principal part in the development of sex. The sex is not generated, but depends upon external influences over the fruit which is in a state of development. And here we have a stage which precedes the separate sex in man, in which stage sexual neutrality must be regarded as normal, where also we find a kind of hermaphroditism.
Whether sex in the human species is determined at conception, or only develops itself afterwards, we must attempt to discover from obstetric experience.
The twins and triplets contained by one chorion are of the same sex, and have a common placenta in which the blood passages of both umbilical cords communicate with one another. On this account, also, many opine that the identity of sex is occasioned by the intermixture of the blood, an opinion regarded, on the contrary, by others, with incredulity, because the intermixture of the blood might very possibly lead merely to a mixture and not to homogeneity, under which circumstances dissimilarity of development would be possible enough. We may here adduce the following facts also (Jhering): The armadilloes produce a number of young in one litter, which are normally developed in a single chorion, and are of the same sex, as in the case with man when twins are developed in one chorion.
Heartless monstrosities (_Acardiaci_) are, in spite of imperfect nutrition (the conditions of proper nutrition by the blood are wanting in the embryo), of the same sex as normal offspring. Now, cases of this kind demonstrate that, in the later periods of development, although the conditions of nutrition are not alike, nevertheless the similarity of sex in the twins is maintained; so the foundation of the future sex would be laid at the period of conception. This rule which proves valid for the twins found in a single chorion, would apply for all human ova in general, because all possess the capacity of attaining their sexual character at conception.
According to Mayrhofer, placentæ are rare in which, where there are two chorions, communication is found between the vessels of the two umbilical cords. In addition to what has been already mentioned, Mayrhofer lays down the law, which we find frequently stated in many quarters, that the older of the two parents has a greater preponderance in favor of the propagation of his, or her, sex. Especially the physical maturity on the man's side enables him to propagate his own sex, either in connection with younger or older women. A superiority on the part of the woman produces girls.