66 Heredity and Eugenics 



feeding the mother on lecithin or by injections of lecithin, 

 the jM'oportion of fenrale births may be increased. His 

 evidence in support of this claim is, however, wholl}' inade- 

 quate, and two independent rej^ietitions of his experiments, 

 made b>' Basil in Italy and by Punnett in England, have 

 given entirely negative results. 



An alternative h}7)othesis concerning the determination 

 of sex has been steadily gaining ground during the last ten 

 years, that sex has its beginning in gametic differentiation 

 and is hnally determined beyond recall in the fertilized 

 egg bv the nature of the uniting gametes. Instructive in 

 this connection is a study of parthenogenesis, reproduction 

 by unfertilized eggs. But before entering upon this, it 

 may be well to review briefly the changes which regularly 

 take place in the egg which is to be fertilized, and compare 

 with this the changes which occur in eggs not to be fertilized. 



In each cell of the ordinary animal there occurs a charac- 

 teristic number of bodies called chromosomes. We do not 

 know that they are any more important than other cell 

 constituents, but we know their history better. These are 

 contained in the nucleus of the cell, and at the time of 

 nuclear division they are found at the equator of the division 

 spindle. There each of them regularly splits in two, and 

 one derivative goes to either end of the spindle, and so into 

 one of the daughter nuclei. Thus each new nucleus has, 

 as a rule, the same chromosome composition as the nucleus 

 from which it was derived. 



But the egg which is to be fertilized undergoes two 

 nuclear divisions in succession, in only one of which do the 

 chromosomes split. In the other division the chromo- 

 somes separate into two groups without splitting, and each 

 group goes into a different cell product. Consequently, 



