1198 



EMBRYOLOGY. 



B 



sinus, as has been indicated, represents the persistent left horn of the sinus 

 venosus. The primitive jugular veins become the internal jugular veins of the 

 adult; the lower part of the right primitive jugular vein forms also the right 

 innominate veins (Figs. 755, 1, 2, 3). 



The foetal circulation has been described in the section on the Blood- Vascular 

 System. 



Development of the Alimentary Canal. As already indicated (page 1157), the 

 primitive alimentary canal is formed, at an early stage, by the enclosure within 

 the embryo of a portion of the blastodermic vesicle, and is seen to consist of three 

 parts, viz. : (1) the fore-gut, within the cephalic flexure and dorsal to the heart ; 

 (2) the mid-gut, opening freely into the yolk-sac ; and (3) the hind-gut, within 



the caudal flexure. The fore- 

 gut and hind-gut end blindly, 

 there being at first neither 

 mouth nor anus (Figs. 756 and 

 757). The formation of the 

 mouth or stomodaeum, and the 

 subsequent communication be- 

 tween it and the cephalic end 

 of the fore-gut, have already 

 been considered ; the manner in 

 which the anus is formed will 

 presently be discussed. 



From the fore-gut are de- 

 veloped the pharynx, oesopha- 

 gus, stomach, and duodenum, 

 and further, as diverticula from 

 the duodenum, the liver and 

 pancreas ; from the hind-gut, 

 the greater part of the rectum, 

 and as a tubular outgrowth 

 from it the hollow stalk of the 

 allantois ; the mid-gut gives 

 ori*gin to the remainder, or 

 longest section, of the alimentary tube i. e., the portion which reaches from the 

 duodenum to the rectum. 



The upper part of the fore-gut becomes dilated to form the pharynx, in relation 

 to Avhich the branchial arches are developed (Figs. 716 and 759); the succeeding 

 part remains tubular, and with the descent of the stomach is elongated to form the 

 oesophagus. Soon a fusiform dilatation, the future stomach, makes its appearance, 

 and beyond this the mid-gut opens freely into the yolk-sac (Figs. 759 and 760). 

 This opening is at first wide, but, as the body-walls close in around the umbilicus, 

 it is gradually narrowed into a tubular stalk, the yolk-stalk or vitello-intestinal 

 duct. At this stage, therefore, the alimentary canal forms a nearly straight 

 tube in front of the notochord and primitive aortse (Fig. 757). From the 

 stomach to the rectum it is attached to the notochord by a band of mesoblast, 

 from which the common mesentery of the gut is subsequently developed. The 

 stomach undergoes a further dilatation, and its two curvatures can be recognized 

 (Figs. 760 and 764), the greater directed toward the vertebral column and the 

 lesser toward the anterior wall of the abdomen, while of its two surfaces one looks 

 to the right and the other to the left. The mid-gut also undergoes great elongation, 

 and forms a V-shaped loop which projects downward and forward ; from the 

 bend or angle of the loop the vitello-intestinal duct passes to the umbilicus (Fig. 

 764). For a time a part of the loop extends beyond the abdominal cavity 

 into the umbilical cord, but by the end of the third month this is withdrawn. 

 With the lengthening of the tube, the mesoblast, which attaches it to the future 



cj o , 



vertebral column and which carries the blood-vessels for the supply of the gut, is 



FIG. 758. Early form of the alimentary canal. (From 

 Kolliker, after Bischoff.) In A a front view and in B an 

 antero-posterior section are represented, a. Four pharyngeal 

 or visceral plates, b. The pharynx, c, c. The commencing 

 lungs, d. The stomach. /, /. The diverticula connected 

 with the formation of the liver, g. The yolk-sac into which 

 the middle intestinal groove opens, h. The posterior part of 

 the intestine. 



