THE PHYSIOLOGY OF DEVELOPMENT 213 



possesses ovaries instead of testes. ''If we now," states 

 Goldschmidt, ** try to formulate a rule which governs this 

 strange seriation ... we find the most important fact that 

 this series is the inverse order of differentiation of the organs 

 in development. The last organs to differentiate in the 

 pupa and the first to be intersexual are the branching of the 

 antennae and coloration of the wings. The first imaginal organ 

 differentiated is the sex gland, and if we apply this law even to 

 the parts of a single organ like the copulatory organ we find 

 it also holds good. 



From these facts Goldschmidt elaborates a hypothesis 

 which may be stated in the following three propositions : 

 (i) that the relative potencies of one or other type of sex- 

 determining reaction-system are not the same throughout the 

 whole course of development ; (2) that genetic factors normally 

 ensure that one or other system predominates at the time 

 when sex- differentiation normally occurs ; (3) that if sex- 

 differentiation can be induced at an earlier or later stage, it 

 may be made to synchronise with the predominance of the 

 system alternative to that which controls differentiation in 

 the normal course of events. To put it in another way, if 

 we represent the potency of the male- and female- determining 

 reactions by ordinates and the time of development along the 

 abscissa, there is usually some stage in the life cycle of either 

 sex where the two curves intersect ; this point generally lies 

 either well before or well after the stage at which sex- differentia- 

 tion actually occurs. Such being the case we should anticipate 

 the possibility of sex-reversal by influencing the growth-rate 

 in forms where the two systems are, as in Lymantria, fairly 

 delicately balanced ; and Goldschmidt has produced female 

 intersexes in pure strains of Lymantria through rearing the 

 embryos at a very low temperature. 



In concluding the foregoing sketch of developmental 

 physiology, one may frankly admit that we are only at the 

 beginning of a scientific treatment of the problem, and no 

 useful purpose is served by understating the difficulties 

 inherent in the subject and the distance which must still be 

 traversed before we can begin to envisage a purely physico- 



