DEVELOPMENT. 



521 



maternal tissues. In the deciduate forms it sometimes grows 

 into the uterine walls round the maternal capillaries, the endo- 

 thelium of which remains throughout (Carnivora), or disappears 

 (Chiroptera) so that the maternal blood is actually in contact 

 with the foetal tissues in lacunae of the trophoblast. In other 

 cases it develops lacunae which become filled with blood through 

 the rupture of the maternal vessels. 



In many lower 

 Mammalia (Marsu- 

 pialia, Rodentia, 

 etc.) the walls of 

 the sac which cor- 

 responds with the 

 yolk-sac of birds de- 

 velops an area vas- 

 culosa which plays 

 an important part 

 in the nutrition of 

 the embryo. 



The early de- 

 velopment of the 

 mammalian embryo 

 is frequently ren- 

 dered apparently 

 abnormal by the 

 precocious forma- 

 tion of that part of 

 the trophoblast and 

 adjacent mesoblast 

 which plays later on 

 an important part 

 in the formation of 

 the placenta. This 

 is notably the case 

 in Homo and his 

 allies, and in those 

 forms which dis- 

 play the so-called 

 inversion of the 

 layers. 



In the foetus, respiration is effected through the placenta 

 and the lungs are functionless. In correspondence with this the 

 circulation of the foetus differs from that of the animal after 

 birth (Fig. 272). From the heart the blood is driven into the 

 descending aorta, which sends off behind two large vessels to 

 the placenta (umbilical or allantoic arteries). The blood, return- 

 ing from the placenta in the allantoic vein, passes in great part 

 through a connecting vessel (ductus venosus Arantii) into the 



Fin. 272. — Diagram of the arrangement of the prin'-ipal vessels 

 in a human foetus (after Huxley). Am amnion; Ao aortic 

 trunk ; Aod descending aorta ; Ax azvgos vein ; C anterior 

 cardinal vein ; Cc common carotid ; " Ce external carotid ; 

 Ci internal carotid ; D ductus venosus arantii ; DO ductus 

 cuvieri : Dv vitelline duct (ductus omphalomeseraicus) ; H. 

 ventricle ; L liver ; N umbilical vesicle (yolk-sac) ; O ompha- 

 lomeseraic (vitelline) artery ; O omphalomeseraic (vitelline) 

 vein ; P lungs ; .S' subclavian artery ; U umbilical (allantoic) 

 arteries with their placental ramifications (U") ; U' umbilical 

 (allantoic) vein ; V auricle ; Vc vena cava inferior ; Vr portal 

 vein ; 1, 2. 3, 4, 5 the arterial arches — the persistent aortic 

 arch is not visible. 



