934 



OF GENERATION EMBRYONIC DEVELOPMENT. 



(Fig. 339, 2, ow"), which is soon joined by a small raesenteric vessel from 

 the intestine (Fig. 339, 3, in). Before this is accomplished, however, the 

 Allantois has been developed, and to this organ the large branches pass 

 from the Iliac Arteries, which are named umbilical arteries; the blood re- 

 turning from it after aeration enters the two Veuse umbilicales which pass 

 forwards and open into the common trunk of the Vense omphalo-raesen- 

 tericee (Fig. 339, i, u'u"). The Vence urabilicales rapidly increase in size, 

 preponderating so much over the only remaining Vena omphalo-mesenterica, 

 into which they originally discharge their contents, that the latter now ap- 

 pears to be merely a tributary branch. As the Liver becomes developed it 

 surrounds the trunk of the umbilical vein (Fig. 339, 3), which soon forms 



FIG. 338. 



FIG. 339, 



om 



?\, 



^V*"' "IT'/. 



FIG. 338. Transverse section made through 

 the body of the Embryonic Fowl, near the 

 umbilicus, on the fifth day of incubation : sh, 

 sheath of the chorda dorsalis; h, epiblast; am, 

 amnios completely closed ; so, secondary aorta ; 

 vc, vena cardinalis; mv, muscular lamina; g, 

 spinal ganglion ; v, anterior root of the spinal 

 nerve ; tip, mesoblast ; up, prolongation of the 

 mesoblast into the abdominal wall (protoverte- 

 bral lamina of Reinak, visceral lamina of Rei- 

 chert); bh, somatopleure, or primitive wall of 

 the abdomen, composed of the external, serous, 

 or epiblastic lamina, and of a layer of meso- 

 blast ; df, internal layer of mesoblast, in which 

 the fibrous membranes of the intestines are 

 developed ; rf, hypoblast. The mass around the 



chorda is that in which the protovertebrte are developed ; it contains the vessels anteriorly, and in front, 

 in the middle line, is prolonged as the mesentery. 



FIG. 339. Diagram of the formation of the Vena Omphalo-mesaraicK and Umbilicales ; 1. At the time 

 of the first appearance of the umbilicales and the commencement of the omphalo-mesaraica?. 2. At the 

 time of the first appearance of the branches to and from the liver, and the diminution of the omphulo- 

 mesenteric vessels. 3, 4. At the period of complete foetal circulation in 1, ompbalo-mesaraic trunk; in 

 2, 3, remains of it; in 4, vein of the yolk-sac alone; om' right and om" left vena omphalo-mc.saniicse; u, 

 trunk of the umbilical vein ; u' right and " left vena umhihValis ; <h; ductus Cuvieri ; ./, jugularis ; c, 

 cardinalis; I, liver; ha, hepatica: advehentes ; hr, hepaticse revehentes; m, meseuterica; da, ductus 

 venosus Arantii ; ci, cava inferior ; p, vena portte ; /, lienalis ; m, meseuterica superior. 



a twofold system of tubes within that gland, the one conveying the blood to 

 tin liver, the Veiuc hepaticse advehentes; the others returning the blood 

 from the gland substance to the umbilical vein again, and constituting the 

 Verne hepaticrc reveheutes. The right umbilical vein now disappears, and 

 the blood returning from the placenta altogether traverses the left vein, 

 which soon takes up a median position ; the remains of the omphalo-mesaraic 

 vessel (om, Fig. 339, 4), together with branches (TO) derived from the intes- 



