240 Professor Clark on a Case 



the vesicle and bowel is gradually diminished, and the vesicle 

 projects in front of the abdomen like an hernial sac. The passage 

 from the vesicle into the bowel, or ductus vitello-intestinalis, is at 

 that part which nearly bisects the length of the future small in- 

 testine. In man the duct becomes gradually capillary, at length 

 ordinarily disappears, and then the only connection between the 

 vesicle and the embryo is by means of blood vessels hereafter to 

 be described. 



Hence it appears that the groundwork of the organs of animal 

 life are formed exclusively on the serous portion of the germina- 

 tive membrane, that the system of organic life is derived originally 

 from the mucous portion, and that the granules of the interme- 

 diate vascular layer exclusively supply the matter from which the 

 blood vessels and their contents originate. It will be seen that these 

 separate portions of the blastoderma afterwards combine to form or- 

 gans upon which depends the advancing evolution of the embryo. 



From the anterior portion of the canal of the heart, arise two 

 vessels, these proceed forwards, surround the anterior extremity 

 of the digestive canal, attain the region of the spine and then 

 unite to form the descending aorta. The aorta loses itself in a 

 small branch on the lower portion of the spine, whilst what may 

 now be more properly considered as its principal continuation 

 leaves the spine at a right angle, reaches the vascular area on 

 the yolk, ramifies there and inosculates with the veins of that 

 membrane and finally enters the terminal vein of the vascular 

 area. This is the first circulation : the aorta cannot be yet seen 

 to ramify within the body of the embryo, and a considerable part 

 of that body has as yet no principal trunk distributed to its sub- 

 stance. The artery which thus proceeds to the umbilical vesicle 

 is the omphalo-mesenteric artery. 



