SEX DETERMINATION 483 
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 asa 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 
