I20 HOW ANIMALS DEVELOP 



chromosomes, while the factors for maleness are 

 carried on the other pairs, which we can call the A 

 pairs to distinguish them. Then the sex which 

 actually develops in an animal depends on the pro- 

 portion between the Xs and the As, and we find 

 that a special mechanism is evolved to regulate this 

 proportion. Thus if the Xs are the female-bearing 

 chromosomes, a female animal is found to have two 

 Xs, making a normal chromosome pair, while the 

 males have only one X, the other X being either 

 completely absent or represented by a special 

 ''empty" Y chromosome, which does not carry any 

 sex factors. The females therefore have a proportion 

 of two Xs to the set of As, while the males have one 

 X to the set of As. This plan makes certain that in 

 the next generation the males and females will be 

 about equal in number. All the eggs must contain 

 one X and one of each of the A pairs, but the sperm 

 will half of them contain an X and half no X (or a 

 Y if the animal belongs to a kind which has them) . 

 In fertihzation there will be equal numbers of 

 animals which get an X in both the egg and the 

 sperm and of animals which get only an X from the 

 egg and none from the sperm. The first kind will 

 be females and the second kind males. Sometimes, 

 however, something goes wrong and the chromo- 

 somes do not divide properly when the eggs and 

 sperm are formed, and in this way animals may get 

 too many Xs in proportion to their As, when they 

 develop into so-called "super-females," or again 

 there may be too few Xs when we get "super- 



