104 



(ESOPHAGUS. STOMACH, AND INTESTINES. 



and gradually widens out into the dilatation which represents the stomach. 1 This 

 organ, which is at first nearly straight (fig. 125, A, Mg\ soon begins to show the con- 

 Fig. 123. SKETCH OF A LONGITUDINAL SECTION THROUGH 



THE ALIMENTARY CANAL OF A HUMAN EMBRYO, 

 SOON AFTER THE DISAPPEARANCE OF THE PRIMITIVE 

 VELUM. (His.) 4 T 



The alimentary canal is shaded throughout; U.K, 

 section of mandibular arch ; R. T, hypophysis ; behind 

 it the remains of the pharyngeal septum ; Lg, com- 

 mencing lung, the future orifice of the larynx being 

 opposite K ; My, stomach ; 7/6, liver ; Nb, yolk stalk ; 

 W, Wolffian duct ; B, blind portion of hind gut ; 

 all, allantois. 



vexity of the greater curvature on the side next 

 the vertebral column, and the concavity of the 

 lesser curvature on the opposite border (fig. 

 125, B, J/#), while the pyloric end becomes 

 tilted away from the vertebral column, pro- 

 ducing the duodenal loop (fig. 125, C, D). 

 Finally the organ becomes turned over on 

 what was previously its right side, which now 

 becomes the posterior surface, and the pyloric 

 extremity being also tilted over, the duodenal 

 loop is thus thrown over to the right side of the 

 abdomen (fig. 12G). The small intestine is also 

 at first quite short and straight, with a wide 

 aperture to the yolk-sac (fig. 125, A, JV&), 

 but gradually lengthens as the communication 

 with the yolk-sac becomes more contracted, 

 and (besides the loop formed by the tilting of 

 the pylorus) develops a long V-shaped loop 

 opposite the attachment of the vitelline duct 

 (fig. 125, C, D, and fig. 127). 



The loop of intestine to which the vitelline duct 

 is attached passes, for a time, into the umbilical 



cord, close to its attachment, enclosed in a protrusion of the peritoneal cavity (fig. 124). It 

 occasionally remains in this situation until late in foetal life. 



Fig. 124. SKETCH OF THE HUMAN EMBRYO OF THE 



TENTH WEEK, SHOWING THE COIL OF INTESTINE IN 



THE UMBILICAL CORD. (Allen Thomson.) 



The amnion and villous chorion have been opened and 

 the embryo drawn aside from them ; T, umbilical vesicle, 

 connected with the coil of intestine, i, by a small, almost 

 linear tube. The figure at the side represents the first 

 part of the umbilical cord magnified ; i, coil of intes- 

 tine ; vi, vitelline-intestinal duct, alongside of which 

 are seen omphalo-mesenteric blood-vessels. 



The mesentery is developed by a thinning 

 out and extension of the mesoblastic tissue 

 which lies between the intestine and the 

 Vertebral column. It forms a continuous membrane along the whole length of the 

 alimentary canal from the stomach to the rectum, although the part attached to the 



1 It has been shown (Balfour, Meuron) in most vertebrates mammals excepted that at a certain 

 period of development the lumen of the oesophagus becomes for a time completely obliterated at its upper 

 extremity* 



