REPRODUCTION 255 



in place of the fetal circulation in consequence of respiratory 

 movements. Owing to the division and occlusion of the umbili- 

 cal cord, blood no longer passes through the umbilical vessels, 

 with the result that the umbilical vein degenerates into a fibrous 

 cord (round ligament of the liver). The hypogastric arteries 

 remain pervious for the first part of their course, as the 

 superior vesical arteries; but the remainder of their course is 

 obliterated and degenerates into fibrous cords. 



The period of gestation during which the embryo is develop- 

 ing in the uterus may be put at 280 days, and probably dates 

 from the first day of the last menstruation. Owing to the diffi- 

 culty of knowing the time of fertilization, the exact period is 

 not known. One of the earliest, and most obvious and most 

 usual, signs of pregnancy is the cessation of menstruation. The 

 cause of the expulsion of the fetus from the uterus is not well 

 known, and it is probable that on account of the exceedingly 

 irritable condition of the uterus a number of causes may exist. 

 Among these have been suggested the pressure on the tissue of 

 "the uterus or on the ganglia of the cervix, and the gradually 

 increasing venosity of the fetal blood. 



The frequency of multiple conceptions is for twins, at a ratio 

 of 1 to 120; for triplets, 1 to 7910; and for quadruplets, 1 to 

 371,126 births. Twins may arise from separate eggs or from a 

 single egg. The presence of a double chorion is diagnostic of 

 the former, and a single chorion of the latter. The separate ova 

 may come from a single Graafian follicle or not. When the off- 

 spring come from separate ova, they may be of separate sexes, 

 and do not necessarily resemble one another; but whenever they 

 come from the same ovum, by a separation of the blastomeres, 

 they are of the same sex, and their personal resemblance is very 

 great. 



The factors that determine sex are very little understood. As 

 a general rule, more boys are born than girls. The sexual organs 

 are differentiated at the eighth week of uterine life, but the 

 trend of modern opinion seems to be that sex is determined in 

 the germ cells at or before fertilization. 



1. In certain worms, eggs of two sizes are produced, of which, 

 the large ones, after fertilization, develop always into females, the 

 small ones into males. 



