REPRODUCTION 



191 



the uterus. Here the embryo proceeds far along in development 

 before birth occurs. (Figs. 133, 134, 169.) 



1. Uterine Development 



In the human body, the attachment of the fertilized egg in the 

 uterus is followed by the very rapid development of what may 

 be regarded as a new uterine lining, profusely supplied with blood 



Fig. 133. — Human egg and sperm. A, four sperm (left); and egg (right) 

 just removed from the ovary, surrounded by follicle cells of the ovary and a 

 clear membrane. The central part of the egg contains metaplasmic bodies and 

 the large nucleus. Superficially there is a clear ectoplasmic region. (Magnified 

 about 400 times.) B, two views of the human sperm, c, centrosome; h, head 

 consisting of the nucleus surrounded by a cytoplasmic envelope; m, ne, middle 

 piece; t, tail or flagellum. (Magnified about 2000 times.) 



vessels. This soon surrounds the embryo and, as pregnancy pro- 

 ceeds, the embryo protected by embryonic membranes projects 

 into, and finally completely fills the gradually increasing uterine 

 cavity. As the uterus enlarges, it exerts increasing pressure on the 

 adjacent organs, and later shifts higher up in the abdominal cavity 

 where the flexible body wall more readily accommodates it. 

 Throughout the entire period of pregnancy the embryo leads 

 essentially a parasitic existence at the expense of the mother's 



