Page:Popular Science Monthly Volume 64.djvu/106

102 fed) there were 51 females and 57 males (and 8 not differentiated). The proportionate number of females to males in all the tadpoles reared in these experiments is not different from that which Cuénot determined in nature. He concludes from his results that the sex of the frog is not influenced by the external conditions (especially of food) to which the tadpoles are subjected.

Pigeons have also furnished some interesting facts in regard to problem of sex. From the time of Aristotle it has been recognized that of the two eggs laid in each batch one generally produces a male and the other a female. Nevertheless numerous exceptions have been recorded in which both individuals were of the same sex. Cuénot himself found in eight sets that in two instances there were two males; in two instances there were two females, and in five instances there was a male and a female. It has been claimed moreover, and the tradition also goes back to the time of Aristotle, that the first egg laid gives rise to a male and the second to the female. Flourens confirmed this fact for eleven sets, and Cuénot found the same result. The meaning of this is obscure, for it may be that a male egg is first set free, or that the conditions to which the first egg that is laid is subjected are such that it becomes a male. The former interpretation may appear to be the more probable, but it is not conclusively established by the facts.

Although many statistics have been brought together in regard to the determination of sex in man and in other mammals there is no convincing evidence showing that external factors determine the sex of the embryo; and, as has been pointed out, there is strong evidence pointing in the opposite direction.

If we turn now to some of the lower animals we shall find that there are a few indisputable cases in which it has been shown that the sex of the individual is predetermined in the egg. It was discovered by Korschelt that two kinds of eggs are produced by a small worm, Dinophilus apatris, and that the larger eggs develop, after fertilization, into females and the smaller into males. The females are about 1.2 mm. long, while the males are only 0.04 mm. long. The males are degenerate in structure; they are less numerous than the females, and live only ten days, whereas the females live a month or more.

A similar difference in the size of the eggs that produce males and females is found in certain rotifers, in Hydatina senta for example. In this species there are three kinds of females distinguished by the different kinds of eggs that they lay. One lays large eggs which without fertilization produce females. Another lays small eggs, less rich in yolk than the last, and these eggs, also without being fertilized,