Page:Popular Science Monthly Volume 64.djvu/102

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98
THE POPULAR SCIENCE MONTHLY

in the case of the armadillo, Tatusa hyhrida of Paraguay. The eight to eleven young of each birth are always of the same sex. This occurs also, it is said, in another species, Tatusa novemcinta. In the latter case it was found by Jehring that all the embryos of one birth are enveloped in a common chorion, although each has its own separate placenta. It is probable that these embryos are the product of a single egg that has become separated during the early stages of segmentation into as many parts as there are embryos produced. That separated blastomeres or cells are capable of giving rise to whole embryos has been demonstrated experimentally in recent years for a number of animals.

The following discovery also bears on the same question. A hymenopterous insect, a chalcid bee, Encrytus fuscicollis, lays one or two eggs in the egg of a caterpillar that is to become the host. The egg of the parasite develops inside the body of the young caterpillar, not into a single embryo, as is the rule, but into a chain of embryos. As many as a hundred embryos may come from the same egg, all united in a common amnion. It has been observed that the bees that emerge from the same caterpillar are frequently of the same sex. Thus in twenty-one observations the progeny was in fourteen cases all of the same sex. In the remaining seven cases both males and females appeared. In the former it is probable that only a single egg had been laid in the egg of the butterfly, and in the latter more than one egg may have been deposited.

One of the earliest and most important of the recent memoirs that have attempted to show that the sex of the individual is determined in the egg is that of Cuénot.[1] This paper deserves first place not only because in point of time it precedes the others to be mentioned, but also because the author has undertaken a considerable number of important experiments that bear on the problem of the determination of sex.

It had been claimed that when young caterpillars are poorly nourished they give rise to a larger number of males, and conversely, when well nourished to a great majority of females. The experiment was first carried out by Landois, and later confirmed by Giard, Treat and Gentry. On the other hand, Riley found that starved caterpillars, as well as those abundantly supplied with nourishment, give both male and female individuals with no greater disproportion in numbers than ordinarily exists. Other observers have recorded similar results. Furthermore, a number of investigators have shown that the sex of the young insect is already determined at the time when it emerges from the egg and even some time before that event. Brocadello's


  1. Bulletin Scientifique de la France et de la Belgique, XXXII., October, 1899.