542 



ORGAN SYSTEMS OF MAN 



placenta 



umbilical 

 cord 



uteru5 



40 weeks 



Fig. 21-9. The full-term fetus shown in its normal position just before birth. 



reach its destination, even though only one 

 is necessary for fertiHzation. Apparently the 

 cooperative effort of many is needed to 

 digest their way through the mucous lining 

 of the female generative apparatus. 



Once the egg is fertilized, it divides sev- 

 eral times en route down the oviduct, and 

 by the time it reaches the uterus it is a ball 

 of cells (Fig. 21-8) and is ready for im- 

 plantation, that is, to become attached to 

 the uterine wall. During the time these 

 things have been happening to the egg, the 

 uterine wall has been preparing itself for 



the reception of the young embryo. When 

 the embryo reaches the uterus, it is actually 

 drawn into the uterine wall because of the 

 receptive nature of the lining itself. Once 

 implanted, the embryo grows rapidly, start- 

 ing as a tiny reddened spot and finally, 

 after approximately 40 weeks, becoming a 

 full term fetus (Figs. 21-9, 21-10). This en- 

 tire process is intricately linked with a com- 

 plex battery of hormones which has been 

 referred to in an earlier section (p. 439). 



During intrauterine life the embryo must 

 develop a complete set of organs — lungs, 



