vii THE STAGES OF LIFE AND DEATH 263 



The omphalo-mesenteric circulation is that which takes place 

 during the first phase of development of the cardio- vascular 

 system, when the heart is represented by a simple pulsating 

 tube which curves in the shape of an S. The arterial extremity 

 of this tube divides into two branches which go to form the 

 primitive aortae which run parallel as far as the caudal end of 

 the embryo. From each of these arteries start five or six 

 collateral branches, two of which, the omphalo-mesenteric arteries, 

 pass out from the embryo through the ventral opening and 

 ramify in the upper half of the umbilical vesicle. This arterial 

 network is continued in a venous network with large meshes 

 ending in the vena or sinus terminalis, which represents the 

 extreme limit of the vascular area. From the extremity of the 

 sinus venosus pass the two omphalo-mesenteric veins, which run 

 together to the venous end of the cardiac tube. In this way 

 the omphalo-mesenteric circulation is established. The blood, 

 forced from the arterial extremity of the cardiac tube, passes 

 through the aortic arches, the thoracic aorta, the two omphalo- 

 mesenteric arteries, the arterial network which traverses the upper 

 half of the umbilical vesicle, and the venous network to the 

 sinus venosus, and returns to the heart by way of the omphalo- 

 mesenteric veins. 



The importance and duration of this first circulation is in 

 mutual relation with the importance and persistence of the 

 umbilical vesicle. In the egg of the fowl and oviparous animals 

 in general it is of great importance and constitutes the nutritive 

 and respiratory apparatus of the embryo during the whole dura- 

 tion of its development. But in the mammals and in man the 

 umbilical vesicle is so slightly developed, persists for such a short 

 time, and contains so little food-stuff, that it can never serve any 

 function indispensable for the development and life of the product 

 of conception. 



The nutrition of the human embryo is supplied much less at 

 the expense of the yolk, which is extremely scanty, than of the 

 nutritive fluids surrounding the ovum in the interior of the 

 womb. These nutritive juices, absorbed by the layers of the 

 blastoderm, penetrate also into the umbilical vesicle, which 

 becomes dilated and enlarged, and from this its vessels draw the 

 materials, probably modified and elaborated, which, through the 

 circulation, provide for the nutrition and development of the 

 embryo. Although the umbilical vesicle very soon atrophies, in 

 proportion to the development of the vascular apparatus of the 

 placenta, yet its two chief vessels persist even after the placenta is 

 well formed. One of the two omphalo-mesenteric arteries becomes 

 a branch of the mesenteric artery, and one of the omphalo- 

 mesenteric veins becomes a branch of the mesenteric vein and a 

 tributary of the portal vein. 



