392 HUMAN PHYSIOLOGY 



courage any great faith in the current stories of prenatal 

 impressions. 



The Sex of the Offspring. Many different views have 

 been held as to the reason why the numbers of individuals 

 of the two sexes turn out to be so nearly equal. The fol- 

 lowing explanation is probably to be accepted. An ovum 

 prior to fertilization is of an indifferent type, it has no 

 tendency to develop a male or a female embryo. The 

 spermatozoa are of two kinds one type will produce a male 

 from the neutral egg while the other will beget a female. 

 If these two sorts of sperm are released from the testes in 

 equal and enormous numbers, the chances will favor a close 

 balance between the sexes. People have greatly desired 

 to control the sex of their children, but the prospect of this 

 is not at all bright. As a matter of fact, boys and girls are 

 not born in precisely equal numbers; about 104 boys are 

 found for 100 girls. It is remarkable that this inequality 

 tends to disappear in any generation, because the mortality 

 among boys is slightly higher than among girls. 



We have said that a fertilized ovum is chemically differ- 

 ent from one which has escaped fertilization. An impor- 

 tant result of this difference is the adhesion of the former to 

 the lining of the uterus. If it is to be retained in the body 

 of the mother and undergo development this attachment is 

 evidently essential. Infertile eggs do not anchor them- 

 selves to this surface, but those that have been impregnated 

 become fixed at some point upon the uterine wall. This 

 lining sloughs and is renewed in connection with each men- 

 strual cycle. It is supposed that this elaborate preparation 

 has to be made each month for the possible implantation 

 of an ovum. 



The immediate result of the union of the male and female 

 nuclei is a remarkable impulse toward cell-division. We 

 have said repeatedly that an unfertilized ovum is only 

 half a cell, and that the same is true of a sperm. When the 

 two have coalesced we have not only a complete cell in 

 place of two halves, but it is a rejuvenated cell, "young" in 

 its active tendencies and in the light of its potential career. 



