576 THE HUMAN BODY 



form sperm preserves the same relation of numbers. Half of the 

 spermatozoa, therefore, will have 23 chromosomes and the other 

 half 24. Since the female germ-cells contain 48 chromosomes each 

 ovum will have 24. In the fertilization of the ovum, if the pene- 

 trating sperm contains 24 chromosomes the offspring will be 

 female; if only 23 the offspring will be male. Obviously this has 

 little practical bearing on the problem of artificial sex determina- 

 tion, except in so far as it shows the futility of attempting to 

 bring it about. It serves, however, to explain a number of facts 

 of inheritance. For example, in certain species of insects all the 

 fertilized eggs give rise to females; the males being derived from 

 eggs that develop without fertilization. This is explained by the 

 fact that only those spermatozoa that have the full number of 

 chromosomes develop to functional maturity. 



Impregnation. The fertilized ovum, which, as we have seen 

 (p. 573), receives the sperm in the Fallopian tube, continues its 

 descent to the uterine cavity, but, instead of lying dormant like 

 the unfertilized, segments (p. 29), and forms a morula. This be- 

 comes embedded in the soft, vascular uterine mucous membrane 

 from which it imbibes nourishment, and which, instead of being 

 cast off in subsequent menstrual discharges, is retained and grows 

 during the whole of pregnancy, having important duties to dis- 

 charge in connection with the nutrition of the embryo. 



Sexual congress is most apt to be followed by pregnancy if it 

 occur immediately after a menstrual period; at those times a ripe 

 ovum is usually in the Fallopian tube, near the upper end of which 

 it is probably fertilized in the majority of cases. There is some 

 difference of opinion as to whether the rupture of the Graafian 

 follicle occurs most frequently immediately before the appearance 

 of the menstrual flow, or towards its close; but the preponderance 

 of evidence favors the latter view. The menstrual process probably 

 is a special preparation of the womb for the reception of an embryo 

 and its nourishment. There is, however, evidence that ova are 

 occasionally discharged at other than the regular monthly periods 

 of ovulation and may be fertilized and cause a pregnancy. 



Pregnancy. When the mulberry mass reaches the uterine cav- 

 ity the mucous membrane lining the latter grows rapidly and 

 forms a new, thick, very vascular lining to the womb, known as the 

 decidua. At one point on this the morula becomes attached, the 



