THE VISCERA 



19 



other the digestive tract. The latter may be regarded as being primarily a 

 straight tube traversing lengthwise the body cavity enclosed by the body wall 

 (figs. 18, 20). The layers of both the visceral and somatic plates which im- 



FiG. 23. — Sagittal Section Showing the Primitive Pericardial and Ccelomic Com- 

 munication, Septum Transversum, Liver, etc., in a Human Embryo of 3 mm. (After 

 Kollmann, from a model by His.) 



Truncus aortae 



Mesocardium posterius 

 Pericardial cavity 



Anterior wall of 

 pericardium 



Septum transversum and floor 

 of pericardium 



Liver 

 Hepatic duct 



Junction'of yolk-sac 



with intestine ^^^^^^iV^}' 



Venous trunk of the heart 



~ Mesocardium laterale 



Ductus Cuvieri 



V. umbilicalis 



V. omphalo-mesenterica 



Ccelomic communication 



Peritoneal cavity 



mediately enclose the body cavity become transformed into a characteristic 

 plem-o-peritoneal membrane. Near the mid-dorsal line, a vertical double plate 

 of peritoneum extends ventrally connecting the somatic (parietal) and visceral 

 layers of peritoneum, and constituting what is termed the mesentery (fig. 20). 



Fig. 24. — Diagram Illustrating the Recession of the |Diaphragm (Septum. Trans- 

 versum) in THE Human Embryo. On the right are indicated the vertebral levels; on the 

 left, the position of the septum transversum in a series of embryos from 2 mm. (XII) to 

 24 mm. (VI) in length, pp, pleuro-peritoneal cavity. (Mall.) 



As development proceeds the digestive tract grows in length more rapidly than the cavity 

 which contains it, and so gradually becomes thrown into numerous coils in the abdominal 

 region, these changes leading to numerous modifications of the original arrangement of the mes- 

 entery. These wiU be described later on in the section on the digestive system. Several out- 

 growths also arise from the primitive digestive tract, to form important organs, such as the lungs, 



