SEX DETERMINATION 453 



SEX DIFFERENTIATION 



It now becomes necessary to distinguish clearly between sex 

 determination and sex differentiation. When we say that by means 

 of a chromosomal mechanism sex is determined, exactly what do we 

 mean ? We answer that the sex of an individual arising from a fertil- 

 ized egg (in the case of parthenogenesis, an unfertilized egg) has been 

 settled. Now as a matter of fact only one thing has been settled irrevo- 

 cably, and that is that one individual will have the chromosome 

 composition characteristic of a male and another individual that of a 

 female. A male is usually an individual that produces spermatozoa 

 and a female one that produces ova. Is it irrevocably settled beyond 

 possibility of reversal that a zygote with the XX chromosome com- 

 position must produce eggs and one with the X composition, sper- 

 matozoa ? This question has apparently been answered by Geoffrey 

 Smith in his work on parasitically castrated crabs and by Richard 

 Goldschmidt on Gypsy moths. In the first case, individual crabs 

 whose testes had been infested by the parasitic cirripede, Sacculina, 

 were gradually changed over in their whole metabolism to such an 

 extent that cells destined to produce spermatozoa produced ova. In 

 the second case, when certain varieties of moth were crossed, all of 

 the germ cells produced females with ova, whereas half of the eggs 

 had the XX and half the X chromosome content. This evidently 

 means that some individuals with the male chromosome character 

 produced eggs. From these results we may be justified in conclud- 

 ing that not even this most fundamental difference of sexes, that of 

 the female producing ova and the male spermatozoa, is irrevocably 

 predetermined at fertilization. 



Lest the reader be confused, however, we hasten to add that under 

 natural conditions of life an individual with the male chromosomal 

 content produces spermatozoa and one with the female chromosomal 

 content produces eggs, and that only rare accidental or unnatural 

 conditions disturb the normal course of events. For purposes of 

 practical genetics we may then define a female as an individual that 

 produces ova and a male as one that produces spermatozoa. 



Secondary sexual characters. Usually males and females differ 

 from each other in many other characters besides the production of 

 eggs or sperm. Often one sex is larger, stronger, more elaborately 

 ornamented and colored than the other and possesses characteristic 

 accessory sex organs whose function it is to facilitate the bringing 

 together of the eggs and the sperm. All of the differences between the 



