An Experimental Study on the Effect of Thyroxin upon Sexual Differentiation in the Fowl

INTRODUCTION. THE work of Hertwig (1912) on the frog has shown that "over-ripening" of the eggs leads to an effective modification of the sex-ratio in the direction of an increased production of males. Adler (1920) examining the thyroids of the larvae arising from such " over-ripe" fertilised eggs found that they are larger than in normal individuals, and that the differentiation of the thyroid gland occurs at an earlier stage in development than that of the gonad. From this study the hypothesis was formulated that the larger and hyperfunctioning thyroid had influenced the processes of sexual differentiation of the gonad of the genetic female in the male direction. Riddle (1914-1916) and Whitman (1919) working with the pigeon demonstrated that from birds made to produce many eggs during the reproductive season there arise more males during the early part (Spring) and more females later. The maleproducing eggs of the early season were found to be smaller than the femaleproducing eggs elaborated later in the Summer. Riddle regards the male-producing ova of the Spring as ova exhibiting a relatively high metabolic rate and the femaleproducing ova of the later season as cells of a lower metabolic rate. Riddle and Fisher (1925) in a more recent paper have shown that the period of most abundant male production is the phase of the reproductive season characterised by the largest thyroid size in the parent, and these authors conclude that successful modifications of sex-differentiation have been effected through changes in the rate of metabolism in the ova and early embryos. The following experiments were designed with a view to determining whether or not definite modifications were produced in the chicken embryo by injections of thyroxin at a stage of development before the visible differentiation of the reproductive organs has occurred. For the present it is assumed that thyroxin in its action provokes an elevation of the metabolic rate.