35 VERTEBRATE REPRODUCTIVE CYCLES 



a process which has been reported commonly in all the 

 lower classes of vertebrates. Although the reason need 

 not concern us here, such a change, when it occurs 

 spontaneously, is always in the direction female to male, 

 and is associated with the sudden appearance of sperm- 

 atic tissue in the ovary. Among fishes such spontaneous 

 reversal has been described in the minnow*^ and in the 

 swordtail,2i and it is sometimes so complete that the 

 pseudo-male animal produces spermatozoa and success- 

 fully fertilizes other females. Among amphibians a 

 series of cases of intersexual frogs and toads has been 

 reviewed by Crew.'* The 'males' which resulted from 

 this change were able to mate normally, but of course, 

 since they were genetically females, all their offspring 

 were females too. In birds sex reversal is also well known, 

 and again Crew'^ has reviewed a series of examples in 

 the case of the domestic fowl. These observations show 

 clearly that as testicular tissue develops male behaviour 

 also appears until finally the birds are able to crow and 

 to copulate with other hens. 



Thus it becomes obvious that male and female 

 behaviour is in no way genetically fixed, but that each is 

 capable of considerable modification by circumstance. 

 Indeed, male behaviour on the part of females is appar- 

 ently normal in some cases, as in the British starling^* 

 and the British robin^^s in autunm. In these birds it has 

 been suggested that its curious appearance at this time 

 is related to a transient excessive secretion of androgenic 

 hormone from the ovary and its biological significance 

 has been discussed. 



In the mammals the sexes are more stable so that true 

 reversals are extremely rare. However, many instances 

 of pseudo-reversal are known, which are caused by an 

 abnormal growth of the adrenal cortex in otherwise 

 normal females. This overgrown and overactive tissue 

 exerts a male-like influence to cause the development of 

 male secondary sexual characters and of male behaviour. 

 Commonly reported cases of sex reversal in man are 



