REPRODUCTION. 



85 



usually given in works on embryology, the student may as weU 

 understand that our knowledge of human embryology in the 

 very earliest stages is incomplete and partly conjectural. The 

 reason of this is obvious: specimens for examination depending 

 on accidents giving rise to abortion or sudden death, often not 

 reaching the laboratory in a condition permitting of trust- 

 worthy inferences. 



It is definitely known that the ovum, which is usually fer- 

 tilized in the oviduct (Fallopian tube), on entering the uterus 

 becomes adherent to its wall and encapsuled. The mucous 

 membrane of the uterus is known to undergo changes, its com- 

 ponent parts increasing by cell multiplication, becoming in- 

 tensely vascular and functionally more active. The general 

 mucous surface shares in this, and is termed the decidua vera ; 

 but the locality where the ovum lodges is the seat of the great- 

 est manifestation of exalted activity, and is termed the decidua 

 serotina; while the part believed to have invested the ovum by 



Fig. 87.— Vascular system of the bnman foetns, represented diagrammatically (Hux- 

 ley), ff, heart; TA, aortic trunk; c, common carotid artery; c', external carotid 

 artery; c", internal carotid artery; *, subclavian artery; v, vertebral artery; 1. 2, 

 3. 4, 5, aortic arcbes; A', dorsal aorta; o. ompbalo-mesenteric artery; dv, vitelline 

 duct; 0', ompbalo-mesenteric vein; v', umbilical vesicle; vp, portal vein; L, liver; 

 V. u, umbilical arteries; «", u'% their endings in the placenta; «', umbilical vein; 

 Dv, ductus venoBus; vK hepatic vein; cv^ inferior vena cava; vU, iliac veins; az, 

 vena azygos; vc', posterior cardinal vein; DC, duct of Cuvier; P, lung. 



