Page:Popular Science Monthly Volume 64.djvu/105

Rh proportion sometimes obtained when tadpoles are reared from eggs artificially fertilized.

Yung found that females of Rana esculenta are twice as numerous as males, while Cuénot found, on the contrary, in a different locality that there were more males than females. It is not known whether this disproportion in the sexes is due to the greater mortality of one sex, or whether there are more eggs of one kind than of the other. The results appear to indicate, however, that external conditions do not have a determining influence on sex, and it seems not improbable, although not completely established, that there may be greater mortality among the male tadpoles than among the females in some species and in certain localities.

Cuénot made a few experiments with the eggs of Rana temporaria. He points out that his results are open to the same grave criticism as are those of his predecessors in that he did not determine the sex of those that died. In one experiment in which the tadpoles were given an abundant supply of vegetable food they suffered greatly from crowding and from insufficient aeration of the water. Their development was retarded and they remained small. Of the 26 frogs that metamorphosed all were females. In another similar experiment there were 3 females and 4 males. In a third experiment the tadpoles were placed in a large aquarium supplied with cold, running water. No food was given, and the tails of the tadpoles were frequently amputated in order to prolong the larval period. There emerged 57 young frogs, of which 33 were females, 29 males, and one hermaphrodite.

In a fourth experiment the tadpoles were separated into three lots. The first were given a vegetarian diet; the second were given only animal food; and the third were put into a large aquarium whose floor was covered with mud, but no food was present. The tadpoles that died were no doubt eaten by their companions and thus a certain amount of food was probably obtained.

The first and the second lots developed at the same rate, but the tadpoles did not reach a large size owing to the small dimensions of the aquarium. They became frogs after two months. Those of the third lot on the contrary were retarded in their development; they remained small and began to die from hunger after the third month. They were then given animal food; they grew rapidly and metamorphosed a month later, i. e., four months after hatching. The frogs were of small size and showed signs of having been poorly nourished. Of the 35 survivors of this third lot 23 were females and 12 were males. Of the 36 survivors of the second lot (with animal diet) there were 14 females and 22 males. Of the 108 survivors of the first lot (vegetarian