REPRODUCTION 



541 



iimplontQlion 



morula 



leovaqe 



oviduct 



ovulation 

 eqq 



fertilization 



voqina 



Fig. 21-8. The human ovary and uterus cut in such a way that the pathway of the fertilized egg can be followed. 

 Note that fertilization occurs in the upper end of the oviduct. One of the oviducts is cut in cross-section in order to 

 show the nature of its lining. 



first; later a liquid-filled cavity appears, in 

 which the egg floats, being attached by a 

 thin stalk to the inner wall. The entire 

 structure is now known as a Graafian fol- 

 licle. When mature it is over 1 centimeter 

 across and is located near the periphery of 

 the ovary once more. The region nearest 

 the outer edge of the ovary thins out and 

 finally splits, allowing the follicular con- 

 tents to be extruded into the body cavity, a 

 process known as ovulation. The tiny egg 

 (about 200 microns across) is carried out 

 with the fluid and is subsequently drawn 

 into the funnel-like terminal end of the 

 oviduct (Fallopian tube), which is lined 

 with beating cilia that create a current 

 directed into the oviduct. Occasionally the 

 e22 gets "lost" in the coelom and never 

 reaches the oviduct. If, however, it should 

 be fertilized while outside the confines of 

 the oviduct, which sometimes happens, the 

 embryo may become attached to any con- 

 venient organ, including the ovary itself, 

 and develop for some time. It usually is 

 unable to go to full term, however, and 

 aborts, causing severe internal hemorrhages 



that may be fatal if immediate care is not 

 given. 



Fertilization 



The union of the egg with a sperm must 

 occur at a rather specific time because 

 neither is long-lived. Fertilization is most 

 apt to occur if sperms are in the vicinity of 

 the egg within a few hours after ovulation 

 and it usually does not occur if more than 

 three days elapse before the union is pos- 

 sible. Ovulation is definitely timed with 

 respect to the menstrual cycle, usually oc- 

 curring about the fourteenth day following 

 the onset of menstruation. There may be 

 rather wide variations in unusual cases, but 

 as a rule a day or two before or after the 

 fourteenth day will find the egg in the ovi- 

 duct where it may be fertflized if sperms are 

 in the vicinity. Sperms deposited in the 

 upper end of the vagina make their way 

 through the cervix (Fig. 21-8) and uterus 

 into the oviducts in a matter of several 

 hours. It is interesting to note that it seems 

 to be essential that mfllions of sperms be 

 supplied at once in order that only one 



