Page:Popular Science Monthly Volume 74.djvu/551

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assume that all the eggs possess a predominating male tendency. There appears to be Justification for this assumption since two polar bodies are formed and a reduction division takes place which may be thought of as partially eliminating the female tendency. Due to the preponderance of female elements, fertilized eggs become females. Unfertilized eggs become males, since the male tendency is undominated.

Aphids produce brood after brood of parthenogenetic females while food and climatic conditions remain favorable. When environmental conditions become adverse both males and females appear between whom eggs are fertilized, which as "winter eggs" resist the elements until the following summer, when they develop into parthenogenetic females. These facts may be thus explained: the original ancestor (stem-mother) of a parthenogenetic series arises from a fertilized egg; the eggs of the parthenogenetic females are all hybrids as to sex. No reduction is thought to take place when the single polar-body is formed. When conditions are favorable, metabolic activities give the ascendency to the female elements; when these become adverse, the male element gains the ascendency in some of the eggs. When males and females appear the sex determinants are segregated in the ova and spermatozoa in equal proportions in each. During maturation in the male aphid, however, Dr. N. M. Stevens, of Bryn Mawr College, and also Dr. W. B. von Baehr, of Germany, have discovered that half the sperm degenerate (these lack the accessory chromosome) and that these are male-producing kind. Hence, since only female-producing sperm remain, all fertilized eggs (sex-hybrids) must develop into females.

In the case of Phylloxera, where many generations of wingless parthenogenetic females appear while environmental conditions remain favorable, and when these become adverse, give way to winged males and females. Dr. T. H. Morgan has discovered a similar degeneration of the male-producing spermatozoa and the loss of a chromosome in the parthenogenetic males. In these cases we must assume that the eggs are intrinsically different (male and female), as they really appear externally, or else, as Correns proposes, that the eggs are potentially male and that in the fertilized egg where both sex tendencies are present femaleness dominates when environmental conditions are favorable to constructive metabolism. But some mechanism or condition must remain whereby an apparently homozygous male may produce spermatozoa bearing a female tendency. It is by no means proved that the whole quota of female tendencies (elements) is eliminated at maturation. The male and female (paternal and maternal) chromosomes are probably promiscuously arranged on the spindle. The persistence of chromosomes bearing female characters may supply the demands for the production of female-producing spermatozoa.

The sex of the offspring then appears to be the result of the inter-