Basis of Sex Determination 211 



made concerning the r&le of the X chromosomes in the 

 determination of sex. We have seen that where sex is 

 determined by two types of spermatozoa one type of 

 eggs is produced which possesses only one X chromo- 

 some. Such eggs might produce males if not fertilized 

 (as they do in bees), but they cannot produce females 

 because for that purpose they must have two X chromo- 

 somes. It has been shown for certain cases, and it may 

 be true generally, that if eggs of this type give rise to 

 parthenogenetic females they may do so because they 

 have for some reason two X chromosomes. Usually 

 such an egg loses one of the X chromosomes in a process 

 of nuclear division (the so-called reduction division) 

 which usually precedes fertilization. If this reduction 

 division is omitted the egg has two X chromosomes and 

 if such an egg develops parthenogenetically it gives rise 

 to a female. These cases do not, therefore, contradict 

 the connection between X chromosomes and sex deter- 

 mination established by cytological observations and 

 breeding experiments, on the contrary, they confirm it. 

 The question remains: How can external conditions 

 bring it about that the reduction division is omitted? 

 To this question no definite answer can be given at 

 present. 



We may in passing mention the well-known observa- 

 tion that twins which originate from the same egg 

 always have the same sex; while twins arising from 

 different eggs show the usual variation as to sex. Twins 



