B. S. Schultze and Ahlfeld, as a result of the investigations regarding twins, also came to the conclusion that the sexual identity of twins depends upon their derivation from a single ovum. If, in accordance with what has been said above, an explanation of the sexual identity of certain twins is to be found in the fact that for such twins there has been a single conception only, and hence the influence, whatever it may be, by which sex is determined acts on both germs at the same time—still the sexual identity of twins in general is remarkable and has not yet been fully explained. The sexual identity in fact occurs much more frequently than appears to correspond to the percentage of twins derived from a single ovum. Von Fricks examined the data relating to multiple births in Prussia during the period of 1826 to 1879 and compiled the following table:
| Per Cent. | Twins. | Triplets. | Quadruplets. | Quintuplets. |
|---|---|---|---|---|
| Boys only | 32.6 | 24.5 | 14.3 | 33.3 |
| Girls only | 30.3 | 22.5 | 19.4 | |
| Boys and girls | 37.1 | 53.0 | 66.3 | 66.7 |
| 2 B. 1 G. | 28.5 | |||
| 1 B. 2 G. | 24.5 | |||
| 2 B. 2 G. | 23.4 | |||
| 3 B. 1 G. | 19.5 | |||
| 1 B. 3 G. | 23.4 | |||
| 4 B. 1 G. | 33.3 | |||
| 3 B. 2 G. | 33.3 |
According to Ahlfeld, of twin births in general, the ratio of those with a common chorion to those with separate chorions is 1 : 8.15. If, however, we wished to explain the frequency with which twins are of identical sex from the occurrence of such twins derived from a single ovum, we should expect to find a very different ratio, namely, 1 : 3.84; that is to say, twins with a common chorion would have to be nearly three times as common as they actually are. From these facts Düsing endeavours to draw the conclusion that external conditions have an influence upon the determination of sex, for very many external conditions are identical in the case of twins; as, for instance, the age of the father, the age of the mother, the nutritive conditions of the ova and of the spermatozoa, the nutrition of the embryo, etc.—all of these would influence both the twins in the same direction. A difference in the sex of twins, on the other hand, might be due to two successive fertilizations.
B. S. Schultze has been led by his embryological studies to the conclusion that there are male and female ova. Thus he believes that sexually identical twins originate from a single ovum with two germinal vesicles, thus assuming that a double fertilization of such ova is possible. But since in such cases the sexes of the resulting twins are always identical the spermatozoon can have no influence upon the determination of sex, but the conditions leading to the development of one sex or the other must pre-exist in the ovum—i. e., there must be male and female ova.
That as a matter of actual fact in some cases the female progenitor exercises an overwhelming influence on the determination of sex, and that the opinion held by so many that this determination depends upon the mother alone is sometimes supported by facts, is shown by the incident recorded by Darwin (“Descent of Man,” Vol. I.), that an Arab mare was delivered seven times successively of a filly, never of a colt, although she was covered by seven different stallions. On the other hand, the circumstance, if it is not to be attributed to pure chance, can also be explained by the constitutional vigour of this mare (Hensen) without adopting the above hypothesis.
Recently Upjohn has maintained that there are two kinds of spermatozoa, male and female; the latter are the commonest, but the former are the more energetic.
An interesting anatomical fact bearing upon this question has been discovered by M. Nussbaum, namely, that in ascaris megalocephala the reproductive glands are indicated already before the separation of the germinal layers; and Nussbaum suggests that this is also the case in all animals, although the proof cannot be obtained in every instance.
Semper has shown also in the case of some of the plagiostomata that long before the beginning of the definitive development of the reproductive organs, the apparently hermaphrodite embryo already possesses at least the tendency toward the constitution of one sex or the other. For in these animals, at a time so early that as yet the reproductive glands exhibit no sexual differentiation whatever, the sexes can nevertheless be distinguished by what appears to be a secondary sexual character. In the female, namely, one ovary only is developed; and very early indeed in the embryos destined to become females we can observe an asymmetrical development of the two germinal furrows. By this characteristic the two sexes can be distinguished far earlier than it is possible to do so by the recognition of a histological differentiation of the reproductive glands.
According to Mayrhofer the prepotency of the male gives rise to the procreation of an excess of males in this way, that physical prosperity of the male probably leads to the generation of boys, whereas prosperity of the female tends to give rise to the generation of girls. Moreover, economizing of the semen by infrequent sexual intercourse tends to originate offspring of the male sex. In this connection Hensen remarks, apropos of the greater excess of male offspring among the Jews: “We might ask whether, in consequence of the comparative sexual continence of the Jews which demands a definite power of endurance on the part of the ova, there may not be effected a certain selection of the ova; and thus we may perhaps explain how it is that in this race an exceptional vigour is somewhat more often to be observed than in other races.” Bock assumes that “thinner semen,” such as results from more frequent sexual intercourse, favours the procreation of the female sex, whereas when intercourse is less frequent a larger number of boys is likely to be born. Janke indicates as two important fundamental principles of scientific physiology that, (1) sexual intercourse represents as it were a contest between the two parties to the sexual act as to which shall transmit his or her sexual influence to the child, the victor in the contest determining the sex of the offspring, and (2) that a crossed inheritance occurs, inasmuch as whichever progenitor proves stronger in this contest transmits to the child the sex other than his or her own. He therefore advises women who long to have a boy to drink a glass of champagne before fulfilling their conjugal duties in order to gain increased sexual vigour.
Fiquet also expresses the opinion that when a vigorous, passionate, and sanguine male progenitor has intercourse with a frigid and phlegmatic woman there will be a preponderant tendency for the offspring to be female; whereas, when the conditions are the opposite of these, the male progenitor being phlegmatic and cold, the female on the contrary sanguine, passionate, and ardent, the offspring will probably be of the male sex.