THE FCETUS. 



1011 



ifh-:' 



are seen in the middle layer, the vessels extending to the surface of the um- 

 bilical vesicle, which gradually shows itself. While the contents of the vitel- 

 line vesicle are undergoing absorption by the embryo, the heart is being completed, 

 the vessels are developing, the allantois is formed, and the placental circulation — 

 which continues until birth — is established. From this time the circulatory 

 apparatus has acquired its definitive 

 disposition. Fig. 575. 



1. Appearance of the heart. — Circu- 

 lation in the umbilical vesicle. — The 

 pleuro-peritoneal cavity of the embryo 

 presents, anteriorly, a diverticulum — 

 the cardiac cavity — in the interior of 

 which the heart is developed. 



From what has been already said, 

 it will be understood that the circu- 

 latory apparatus is adapted to take the 

 part, successively, of the umbilical 

 vesicle and the placenta ; and from this 

 two great systems arise — the umbilical 

 or omphalo-mesenteric circulation, and 

 the allantoid circulation. 



As Dareste has it, the heart is 

 primarily double. Its two portions are 

 formed on each side by a folding in- 

 wards of the fibro-intestinal lamina 

 and its endothelial lining. The two 

 cardiac cavities open into each other 

 by the absorption of the walls joined 

 as a septum, and the simple heart 

 assumes the form of a looped tube 

 inclined to the right, as in Fig. 577. 



As soon as it shows itself, the 

 heart contracts and dilates alternately, 

 the movements being very slow, though 

 they gradually become quicker. To- 

 wards the twelfth day, the central 

 organ of the circulation has the 

 form of a contractile cylindrical tube. 

 From its anterior part spring two 

 branches — the aortic arches {arcus 

 aortce) — which are directed towards 

 the head of the embryo, and are 

 afterwards inflected downwards and 

 backwards. 



The aortic arches join together to constitute the single aorta, which, in its 

 turn, divides into two trunks— the arterice vertebralis, or primitive aortcp. These 

 vessels pass along the lower surface of the embryo, parallel to each other, and 

 furnish during their course four or five divisions— the omphalo-mesenteric arteries 

 —which ramify in the area germinativa, and open into a limitary vein named 

 the sinus (or vena) terminalis. From the network of the area and the sinus 



DIAGRAM OF THE FORMATION OP THE VENJE 

 OMPHALO-MESENTERICiE AND UMBILICALES. 



1, At the time of the first appearance of the um- 

 bilicales, and the commencement of the om- 

 phalo-mesenteric£e ; 2, at the time of the first 

 appearance of the branches to and from the 

 liver, and the diminution of the omphalo- 

 mesenteric vessels ; 3, 4, at the period of com- 

 plete fcetal circulation in 1, omphalo-mesenteric 

 trunk ; in 2, 3, remains of it ; in 4, vein of the 

 yolk-sac alone ; om', right, and om", left vena 

 omphalo-mesentericse ; u, trunk of the um- 

 bilical vein ; u', right, and u", left vena um- 

 bilicalis; c?c, ductus Cuvieri ; y, jugularis; c, 

 cardinalis ; /, liver; ha, hepatica advehentes; 

 hr, hepaticffi revehentes ; m, mesentericae ; da, 

 ductus venosus Arantii ; ci, cava inferior; p, 

 vena portee ; I, lienalis ; m, mesenterica su- 

 perior. 



