EMBRYOLOGY. 267 



The hypoblast gives rise to the epithelial lining of the alimentary 

 canal and its glandular appendages, the liver and pancreas, and the 

 epithelium of the respiratory tract. 



Germinal Area. At about this period there is an accumulation 

 of cells at a certain spot upon the surface of the blastodermic 

 membranes, which marks the position of the future embryo. This 

 spot, at first circular, soon becomes elongated, and forms the primi- 

 tive trace, around which is a clear space, the area pellucida, which 

 is itself surrounded by a darker region, the area opaca. 



The primitive trace soon disappears, and the area pellucida be- 

 comes guitar-shaped ; a new groove, the medullary groove, is now 

 formed, which develops from before backward, and becomes the 

 neural canal. 



Dorsal Laminae. As development advances, the true medullary 

 groove deepens, and there arise two longitudinal elevations of the 

 epiblast, the dorsal lamina, one on either side of the groove, 

 which grow up, arch over, and unite so as to form a closed tube, 

 the primitive central nervous system. 



The chorda dorsalis is a cylindric rod running almost throughout 

 the entire length of the embryo. It is formed by an aggregation of 

 mesoblastic cells, and is situated immediately beneath the medul- 

 lary groove. 



Primitive Vertebrae. On either side of the neural canal the cells 

 of the mesoblast undergo a longitudinal thickening, which develops 

 and extends around the neural canal and the chorda dorsalis, and 

 forms the arches and bodies of the vertebrae. They become divided 

 transversely into four-sided segments. 



The mesoblast now separates into two layers : the external, joining 

 with the epiblast, forms the somatopleura ; the internal, joining 

 with the hypoblast, forms the splanchnopleura ; the space between 

 them constitutes the pi euro -peritoneal cavity. 



Visceral Laminae. The walls of the pleuro-peritoneal cavity are 

 formed by a downward prolongation of the somatopleura (the 

 visceral lamina), which, as they extend around in front, pinch off a 

 portion of the yolk-sac (formed by the splanchnopleura), which be- 

 comes the primitive alimentary canal ; the lower portion, remaining 

 outside of the body cavity, forms the umbilical vesicle, which after a 

 time disappears. 



