1144 THE REPRODUCTIVE ORGANS 



(c) one in which the chromosomes are evenly reduced and identical 

 in appearance. If either one of the first two varieties fertilizes the 

 egg, a male results, whereas that variety which possesses the identical 

 chromosomes gives rise to a female. 



In man, the number of the chromosomes is given as 23 and 24 

 respectively. The first type of spermatozoon gives rise to males and 

 the second to females. Consequently, the segmentation nucleus 

 must contain 47 chromosomes in the first case and 48 in the second. 

 These facts, however, explain sex only and do not show why the 

 number of male children born at full term is greater than that of the 

 females. The ordinary relationship of 106 males to 100 females rises 

 to 130:100 in women whose first children were born after they had 

 reached their thirtieth year, and to 140 : 100 in women whose first 

 children were born after their fortieth year. The statistics gathered 

 more recently confirm the old view that a greater number of males are 

 born during times of war. 



Parturition. — The fetus is fully developed and ready to be expelled 

 280 days after the first day of the last menstrual period, but this date 

 is only approximate, because normal children are also born as early as 

 240 and as late as 320 days after the day just specified. In fact, even 

 these extremes are exceeded sometimes. These discrepancies are due 

 in part to differences in the rate of development, and in part to our 

 inability of exactly determining the time of the fertilization of the 

 ovum. Evidently, this calculation cannot be based upon the day 

 of the coitus, because fertilization takes place at variable intervals 

 after insemination. Such differences have also been noted among the 

 domestic animals in which the duration of pregnancy is usually deter- 

 mined in accordance with a single coitus. In the cow, for example, 

 it is estimated at 280 days, with extremes of 240 and 310 days, and 

 in the mare at 366 days, with extremes of 307 and 412 days. 



Labor consists essentially in the development of a driving force 

 which is capable of separating the fully formed fetus from the mother 

 without injury to either participant. Particularly at this time the 

 female genitals display their dynamic qualities most advantageously, 

 and this is true especially of the uterine musculature which plays the 

 principal part in this process. Already during pregnancy the woman 

 experiences intermittent contractions of this organ which, however, 

 do not give rise to unpleasant sensations. At the time of labor, these 

 contractions increase in intensity and are associated with a distinct 

 pain which possesses a peculiar bearing down character, i.e., they 

 begin in the sacral region and slowly pass to the abdomen and to- 

 ward the thighs. To begin with, they recur at intervals of from 15 

 to 30 minutes, but later on as frequently as every 2 minutes. 

 They may then last for 60 to 90 seconds. The dilatation of the cervix 

 having been accomplished, the climax of these 'labor pains" is reached 

 at the time when the head distends the vulva. In many cases, how- 



