Page:Popular Science Monthly Volume 74.djvu/549

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the time of emergence from the egg and deposition of her first egg determines which kind of egg she shall subsequently produce. The well-nourished females produce female eggs, the poorly nourished produce male eggs. Sex, however, is determined while the ovum is still in the ovary. When the eggs have once been formed no subsequent change of food or temperature can alter the kind of eggs that are produced. Phylloxera vitifola, the plant louse that lives on the roots of the grapevine, also produces large and small eggs, developing parthenogenetically (without fertilization) into females and males, respectively.

The second position is that taken by Strasburger, Castle, Wilson and others who have derived their facts largely from the group of insects. These investigators are inclined to believe that there are two kinds of eggs as there are two kinds of spermatozoa, and that sex is the outcome of an interplay or struggle of the sex determinants of these elements and the dominance of one or the other sex. According to these biologists all animals are sex-hybrids; either sex is potentially present originally, but by reason of the dominance of one or the other sex-determinant the particular sex becomes patent and makes the animal definitively male or female. Dominance may be due, in many cases, as is indicated by some of the Hemiptera, to the presence of one or several extra and specific chromosomes—"idiochromosomes," "X-element"

(Wilson). According to this view fertilization is selective, i. e., a female egg is fertilized only by a male sperm, and vice versa. There are many observations and ascertained facts to support this position.

The case of the large "walking-stick" insect (Aplopus mayeri) or "devil's riding horse" of Loggerhead Key, Dry Tortugas, Florida, which I studied in 1907, illustrates the position under discussion. Similar facts had been reported previously for many insects by various American cytologists, notably Professor McClung, of the University of Kansas; Professor Montgomery, of the University of Pennsylvania, and Professor E. B. Wilson, of Columbia University. Aplopus mayeri (named after Dr. A. G. Mayer, director of the Biological Laboratory of the Carnegie Institution of Washington at Dry Tortugas, Florida) produces two kinds of spermatozoa differing in the number of chromosomes (rod-like bodies supposed to be the vehicles of the hereditary qualities) that the two classes possess. One half of the spermatozoa hold 17 chromosomes and the other half 18, the additional one being large and V-shaped (called by McClung the "accessory chromosome"). The somatic cells of the male contain 35 chromosomes, the somatic cells of the female, 36 chromosomes. When the eggs mature, the 36 chromosomes are reduced, by fusion in pairs and a subsequent double division, to 18 chromosomes. Now at fertilization when an egg of 18 chromosomes unites with a spermatozoan of 18 chromosomes an organism whose cells contain 36 chromosomes results, and this is a female;