SEX DETERMINATION 227 



effect spreads more widely and both gonads are affected; again it is the 

 male which suppresses the development of the female, unless a very 

 small male is grafted with a very large female, when it may be the 

 female which takes the lead. 



This beautiful work brings the male and female substances, postu- 

 lated by Goldschmidt, out of the realm of the hypothetical into a sphere 

 where they should be capable of biochemical analysis. The two main 

 gaps remaining in the embryological part of the story are: (i) can the 



9 9 



SALAMANDERS 



Fig. 105. Sexual Development of "Parabiotic Twins" in Amphibia. — The 



animals are grafted together in early embryonic stages and allowed to develop 

 until the sexual differentiation is well advanced. In pairs of toads, partners of 

 different sexes have no effect on one another. In frogs, a male suppresses the 

 differentiation of the ovary nearest to it, and may convert part of it intoatestis-like 

 organ. In salamanders and newts, the male dominates if it is larger or the partners 

 equal in size; usually it entirely suppresses the ovarian development of its 

 "co-twin"; it may itself suffer some retardation in the development of its testes. 

 If a male of a small species is grafted on to a female of a large species, the female 

 dominates and converts the male organs into ovaries. In the diagram testes are 

 represented black, ovaries white. Note Bidder's organ (ovarian) in the male toad. 



(After Witschi.) 



somatic part of the gonad induce neutral cells to become germ-cells or 

 can it only decide what type of development shall be followed by cells 

 which are already determined as germ-cells ? (2) the mesodermal part 

 of the gonad is itself presumably induced by the primary organization 

 centre of the gastrula; if one placed non-gonad mesoderm of a male 

 into the gonad forming region of a female it would almost certainly 

 form a gonad, but we do not know whether this would be a male or a 

 female gonad. 



In mammals and birds the mechanism of sex determination is 

 probably essentially similar, depending on the stimulation of the cortex 

 or medulla by the F or Af substances. The differentiation of the sex 

 organs other than the gonads (ducts, external genitalia, etc.) and of the 



