664 THE PHYSIOLOGY OF REPRODUCTION 



fixed at the time of fertilisation. In the case of the lamprey, 

 somewhat similar conclusions have' been arrived at by Okkelberg^ 

 in an important paper. In this animal the germ-cells of each germ 

 gland are seen at a very early stage to be of two kinds, one set 

 showing a tendency to divide rapidly, while the other shows a 

 tendency to grow, dividing very seldom. The former, he believes, 

 have a male, the latter a female, potentiality. The relative pro- 

 portion of anaboHc and katabolic cells determines whether the 

 larva becomes <? or ?. The lamprey, like the frog, thus carries the 

 potentiality of both sexes, and its sex is not irrevocably fixed at 

 the time of fertilisation. Other cases of apparent sex-reversal are 

 considered in a later section of this chapter. 



. The experiments of Treat ^ and other observers who attempted 

 to show that the sex of caterpillars could be determined artificially 

 by regulating the supply of food should perhaps be disregarded, as it 

 has since been shown that the sex in those animals is probably 

 established at the time of hatching. Furthermore, experiments by 

 Briggs ^ and other investigators have failed to support the hypothesis 

 that the proportion of the sexes can be altered by modifying the 

 diet, while Kellogg * has shown that with the silkworm moth, sex 

 is determined as early as immediately after the first larval moulting, 

 this conclusion being based on ah examination of the rudimentary 

 reproductive glands. 



According to Cu6not,^ the essential organs of reproduction in the 

 maggots of flies are not differentiated into ovaries or testicles until 

 a late period of larval development. It seemed possible, there- 

 fore, that in these animals the sex could be mofiified by the 

 conditions of nutriment or other external factors. Cuenot found, 

 however, that the proportion of the sexes was not materially affected 

 by the supply of nourishment, although the maggots were fed upon 

 different kinds of food, some being given brain, suet, and a little 

 meat, some a large supply of putrefying flesh, while others were 

 relatively starved. 



Among bees and other hymenopterous insects the nutriment 

 appears to be the main factor determining the difference between the 

 two kinds of females (workers and queens). A worker larva can be 

 made to develop into a queen by supplying " royal food," that is, food 

 which is given to young queens. In the worker the female generative 



' Okkelberg, " The Early History of the Germ Cells in the Brook Lamprey, 

 Entospeuus WUderi (Gage), up to and including the period of Sex-differentiation," 

 Jowr. of Morph., vol. xxxv., 1921. 



2 Treat, "Controlling Sex in Butterflies," American Natwalist, vol. vii., 1873. 



^ Briggs, "Notes on the Influence of Food in Determining the Sexes of 

 Insects," Trans. Entom. Soc, London, vol. i., 1871. 



* Kellogg, "Notes on Insect Bionomics," Jow. of Exp. Zopl., vol. i., 1904. 



^ Cu6not, loc. cit. 



