THE FACTORS WHICH DETERMINE SEX 633 



be found. Starvation experiments were undertaken, and in 

 these also thelytokous females which had hitherto " bred 

 true " continued to do so. Punnett concludes that neither 

 temperature nor nutrition has any influence in determining 

 the production of arrenotokous females. On the contrary, it 

 is the property of certain females to produce arrenotokous 

 females in a definite ratio, and the property of others to 

 produce none. 



Theories which assume that the Gametes are themselves Sexual. 

 — Many biologists have entertained the conception that the 

 gametes are themselves sexual, and a number of facts have 

 been adduced which give very strong support to this idea. Some 

 of these have already been mentioned, but probably the 

 strongest evidence in favour of this generalisation is that relating 

 to the existence of sexually differentiated spermatozoa. 



It has been known for a long time that two kinds of sperm 

 exist in the snail Paludina, a hair-like form and a worm-like 

 form, but it is commonly beheved that only the former is func- 

 tional. Dimorphic spermatozoa have also been discovered in 

 various other animals, but the differences between the two 

 kinds vary greatly. ^ 



Henking2 made the discovery that in the bug, Pyrrhocoris, 

 half of the spermatozoa differ from the other half in possessing 

 an additional chromosome. McClung ^ was the first to suggest 

 that the difference between the two sorts of spermatozoa in this 

 insect was connected with sex-determination, and that those 

 which contained the accessory chromosome produced males and 

 that those without it produced females. The "last assumption 

 has, however, proved to be incorrect, since Wilson * found that 



' A list of species in which dimorphic forms of spermatozoa have been 

 recorded (down to 1902) is given by Beard, loc. cit. 



^ Henking, " Untersuchungen ueber die ersten Entwicklungsvorgange in 

 den Eien der Insekten," Zeitschr. f. wiss. Zool., vol. xlix., 1890, and vol. li., 

 1891. 



' McClung, "The Accessory Chromosome Sex Determinant," Biol. Bull., 

 vol. iii., 1902. 



* Wilson, " Studies on Chromosomes," Jour, of Exp. Zool., vols. ii. and 

 Hi., 1905-6 ; vol. vi., 1909. " Note on the Chromosome Groups of Meiapodius 

 and Banusa," Biol. Bull,, vol. xii., 1907 ; "The Supernumerary Chromosomes 

 of Hemiptera," Science, vol. xxvi., 1907; see also Stevens, "Studies in 

 Spermatogenesis," Part I., 1905, and Part II., 1906, Carnegie Institution 



