CHAP. XLII.] OF THE LIVER AND PANCREAS. 607 



to be formed originally from the most superficial ceils of the yolk. 

 The mucous membrane is developed from a thin layer of smaller 

 cells of the yolk in the interior. Between the outer layer, which 

 becomes converted into the muscular coat, and the inner layer, 

 which constitutes the mucous membrane, a glandular layer is 

 formed. From this tube the different parts are gradually evolved. 

 The omphalo-mesenteric duct is connected with the lower part of 

 the small intestine, just previous to its junction with the large in- 

 testine. The original connection with the umbilical vesicle is 

 sometimes marked by an elongated pouch, or diverticulum, per- 

 sistent in the adult. The original yolk-cells, contained in the 

 cavity of the intestine, slowly disappear. The length of the 

 small intestine gradually increases, until it assumes its mature 

 form. 



The stomach, at an early period, is not wider than the rest of the 

 canal and its limits are not to be distinguished. 



Originally, the tube of the intestine is completely closed, both 

 at the mouth and anus. The membrane is gradually removed, 

 and an opening formed. In cases of imperforate anus there is no 

 opening, in which condition an operation is necessary, as soon as 

 possible after birth. 



Development of the Liver, and Pancreas. The precise mode 

 of origin of the liver in the embryo has not yet been ascer- 

 tained with certainty. Some observers hold that this- large gland 

 is originally formed upon a diverticulum of the intestine, while 

 others have concluded that it is developed from a distinct and 

 separate mass of blastema. In the chick the first rudiment of this 

 organ may be discerned between the fiftieth and sixtieth hour, and 

 is described by Kemak as consisting of two sets of cells an ex- 

 ternal one, continuous with the external surface of the intestine, 

 and an internal layer, composed of epithelium, and lining the Sac, 

 which ultimately becomes divided, so as to form the ducts. From 

 the epithelial lamina the columns of liver cells are formed ; these 

 extend into the outer lamina, branch and anastomose, and include 

 in the meshes thus produced, the cells of the outer surface, from 

 which the vessels, nerves, and areolar tissue of the gland are 

 developed. 



Miiller describes the liver, as formed on the fourth day of incu- 

 bation, by a conical protrusion of the intestinal tube. The walls 

 of the protrusion become very thick, and in their substance the 

 ducts ramify. 



According to Reichert, the liver and pancreas in the embryo 



