FOLDING OF THE PERITONEUM. 



7& 



than tliat which would make them directly enclose the colon. On the 

 other hand, the farther elongation of the omental sac and the whole 

 disposition of the peritoneum, with respect to the colon, must be 

 regarded as having taken place after the assumption by the great intes- 

 tine of its permanent position. 



Fig. 579. — Sketch op the HujijiN Fig. 57 



Embryo of the Tenth Week, show- 

 ing THE Coil of Intestine in the 

 Umbilical Cord. (A. T.) 



Tlie amnion and villous chorion have 

 been ojiened and the embryo drawn aside 

 from them ; r, the umbilical vesicle or 

 yolk-sac placed between the amnion and 

 chorion, and connected with the coil of 

 intestine, i', by a small or almost linear 

 tube ; the figure at the side represents 

 the first part of the umbilical cord 

 magnified ; i, coil of intestine ; v i, 

 vitello-intestinal duct, alongside of which 

 are seen omphalo-mesenteric blood-ves- 

 sels. 



The occuiTence of umbilical hernia in its various degrees may be 

 referred to the persistence of one or other of the foetal conditions in 

 which a greater or less portion of the intestinal canal is contained in 

 the umbilical cord ; and it has been shown that the most common form 

 of abnormal diverticula from the small intestine is connected with the 

 original opening of the ductus vitello-intestinalis into the ileum. 



DEVELOPMENT OP THE LIVER, PANCREAS, AND SPLEEN. 



The Liver. — The liver is one of the earliest formed abdominal 

 organs. It consists at first, according to most observers, of two solid 

 masses in connection with the lower surface of the duodenal portion of 

 the alimentary canal. Schenk, however (Lehrbuch, p. 93), states that 

 the blastemic mass of the liver is single. A hollow cavity soon appears 

 within the mass, which is the commencement of the main excretory 

 duct (ductus choledochus communis). This cavity is lined by hypo- 

 blastic epithelium ; and, according to the commonly received view, is 

 produced as a diverticulum of the hypoblast of the intestine. Through 

 the mass, but at first unconnected Avith its substance, there passes the 

 main stem of the veins from the umbilical vesicle and allantois (um- 

 bilical vein or meatus venosus). 



In the rudimentary mass composing the liver there are soon observed 

 a number of solid cylinders of blastemic cells which branch out from 

 the hypoblast into the mesoblast, and as these come to unite together 

 by their ends, they at last form a network of solid cords with which 

 the hypoblastic diverticula are connected. In the meantime blood- 

 vessels are developed in the mesoblast lying between the cylinders, 

 which vessels become united as branches with the umbilical vein pass- 

 ing through the liver. Hollow processes also extend themselves from 

 the hypoblastic diverticula and stretch into the solid cylinders of the 

 hepatic parenchyma ; but the greater part of this remains solid for a 

 time, consisting of reticulated strings of cells between which there is 

 nothing but blood-vessels. 



According to some embryologists, as Schenk, the hypoblast forms no 



