Page:Popular Science Monthly Volume 74.djvu/554

550 dieciousness must be regarded as unit characters, since they behave in crossing according to Mendelian laws. Hermaphroditism probably has its chromosome determinant as also dieciousness, hence the cytological study of the germ cells of hermaphrodite forms will hardly supply the clue to the cause of sex as it was until recently believed to do.

In those insects where there is a dimorphism of spermatozoa, consisting in the presence of an accessory chromosome in one half of the gametes, no obvious difference appears among the eggs. There are no real grounds for a division of the ova into those with male character and those with female character. In the light of Correns's recent observations, the eggs had better all be regarded as possessing the same sex tendency, and this probably female in nature. The spermatozoon with the accessory chromosome need then only be thought of as possessing the female tendency, and the one that lacks it, as possessing the male tendency, and the facts are brought into line with the results of Correns and admit of the same interpretation. In the case where large and small eggs are produced, as by Dinophilus apatris, it is conceivable that there is selective fertilization, i. e., while both tvpes of egg may be female in tendency, only the larger admit of fertilization by a female-producing spermatozoon.

The present status of the case concerning the determination of sex, as well supported for a large class of plants and animals, appears to be that sex is determined by the spermatozoa (or pollen grains)—which are male and female in the proportion of 1:1—and at the instant of fertilization. But surely it would be the utmost folly to hold on the basis of so comparatively few facts, that this explanation applies universally. Nature arrives at similar ends by devious and divers ways and it is not inconceivable that sex has been attained by several paths, and is now determined in different modes and at different times in the different groups of animals and plants.