DETERMINATION AND INHERITANCE OF SEX 545 



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 pro- 

 duce male eggs. Sex, however, is determined while the ovum is still in 

 the ovary. Wlien 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 grape- 

 vine, 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 chromo- 

 somes (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 chro- 

 mosomes are reduced, by fusion in pairs and a subsequent double divi- 

 sion, to 18 chromosomes. Now at fertilization when an egg of 18 

 chromosomes unites with a spermatozoan of 18 chromosomes an organ- 

 ism whose cells contain 36 chromosomes results, and this is a female; 



