THE FINAL ADJUSTMENTS II7 



neural tubes in newt embryos. It seems as if they 

 were a group of skeleton keys each of which can 

 unlock several different doors. 



Sex 



The development of sex is interesting not only in a 

 general way, because sex is such an important ele- 

 ment in our make-up, but also from the strictly 

 embryological point of view it raises quite special 

 and remarkable problems. When an animal develops, 

 not only must the sex-cells, eggs and sperm, be 

 elaborated, but the animal assumes a particular 

 sexual character which affects its whole constitution, 

 making it either male or female. These two phases 

 of sexuality are to some extent distinct, at least in 

 the way they arise. 



The most prominent fact about the sex-cells is that 

 they are capable of starting off ^ new development 

 into another organism. One fertilized tgg produces, 

 among other things, another ^gg which can be 

 fertilized and so on ad infinitum. This behaviour was 

 emphasized in the Theory of the Germ-Track, which 

 states that there is a continuous succession of germ- 

 cells, the later ones derived from the earlier ones by 

 division and reunion in fertilization, so that they are 

 really all parts of the same living substance. This 

 line of germ-cells represents, then, an immortal piece 

 of living matter, which may increase in size but 

 need never die. The thing which does die is the 

 individual animal body which the germ-cells make 

 to provide themselves with a temporary house. 



