THE FCETUS. 



1041 



ifh' 



-^/V: 



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. Appenranre of the heart. — Cirni- 

 lation in the umbilical vesicle. — The 

 pleuro-peritoneal cavity of the embryo 

 presents, anteriorly, a diverticulum — 

 the cardiac caintij — 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 oraphalo-mescnteric circulation, and 

 the allantoid circulation. 



As Dareste has it, the heart is 

 primarily ilouble. 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 ])ecome 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 (areas 

 aorftx) — which are directed towards 

 the head of the embryo, and are 

 afterwards inflected downwards and 

 backwards. 



The aortic arches join together to constitute the sinf/Ie aorta, which, in its 

 turn, divides into two trunks— the arterice vertebralis, ov primitive aorta'. These 

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

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

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

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



DIAGRAM OF THE FORMATION OF THF. VENJE 

 OMPIIALO-MESKNTERICE AND UMBILICALES. 



1, At the time of the first ;'ppearance of the um- 

 bilicales, ami the commencement of the om- 

 phalo-meseiiterica' ; 2, at the time of the first 

 appearance of the branches to and from the 

 liver, and the diminntiou of the omphalo- 

 mesenteric vessels; 3, 4, at the |etiol of com- 

 plete foefal circulation in 1, omphaio-mesenteric 

 ' runk ; in 2, 3, remains of it ; in 4, vein of the 

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

 oinphalo-mesenterica; ; m, trunk of the um- 

 bilical vein ; u', right, and »<", left vena nm- 

 bilicalis; rfc, ductus Cuvieri ; y, jui^ularis ; c, 

 cardinalis ; /, liver; ha, hepatica advehentes; 

 hr, hepaticse revehentes; m, niosentericae ; da, 

 ductus venosus Arantii ; ci, cava inferior; p, 

 vena porta; ; /, lienalis ; m, niesenterica su- 

 perior. 



