THE FINAL ADJUSTMENTS II9 



who wishes to can now make certain of the im- 

 mortality of part of himself as an actual material 

 living thing by endowing a staff of experts to keep 

 the cultures going. Each culture, of course, only con- 

 tains a very small piece of tissue, a few hundredths 

 of an inch across, so that it is only rather a piecemeal 

 immortality which science offers. But at the same 

 time each of these little cultures is growing, doubling 

 its volume in 2 days, and the tip of your little 

 finger would grow to the size of the world in 

 less than 6 months. 



We must now turn to the second question; what 

 decides whether the germ-cells, and the whole 

 animal, shall be male or female? Actually two groups 

 of factors work together in this sex-determination, 

 and sometimes one is the more important, sometimes 

 the other. One set of factors are given by the heredi- 

 tary make-up of the embryo and the other set are 

 particular conditions which arise during develop- 

 ment. The hereditary basis is probably always 

 present even where its influence is overcome by the 

 factors which arise later. The important thing to 

 notice about the hereditary determination of sex is 

 that all animals inherit tendencies towards both 

 maleness and femaleness, sometimes with one pre- 

 ponderating, sometimes with the other. The heredi- 

 tary factors or genes for maleness and femaleness are 

 usually carried on different chromosomes. One 

 scheme is to have all the genes for one kind of 

 sexuality, say femaleness, carried on one pair of 

 chromosomes, whi(:h are then called the X- 



