156 . HUMAN PHYSIOLOGY. 



vitellus is divided into a great number of cells, each of which contains a 

 nucleus and nucleolus. 



The peripheral cells of this "mulberry mass" then arrange themselves 

 so as to form a membrane, and as they are subjected to mutual pressure, 

 assume a polyhedral shape, which gives to the membrane a mosaic appear- 

 ance. The central part of the vitellus becomes filled with a clear fluid. 

 A secondary membrane shortly appears within the first, and the two to- 

 gether constitute the external and internal blastodermic membranes, 



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 \ht primitive trace, around which is a 

 clear space, the area pelludda, which is itself surrounded by a darker region, 

 the area opaca. 



The primitive trace soon disappears, and the area pellucida becomes 

 guitar-shaped; a new groove, the medullary groove^ now formed, which 

 develops from before backward, and becomes the neural canal. 



Blastodermic Membranes. The embryo, at this period, consists of 

 three layers, viz.: the external and internal blastodermic membranes, and a 

 middle membrane formed by a genesis of cells from their internal surfaces. 

 These layers are known as the epiblast, mesoblast and hypoblast. 



The Epiblast gives rise to the central nervous system, the epidermis of 

 the skin and its appendages, and the primitive kidneys. 



The Mesoblast gives rise to the dermis, muscles, bones, nerves, blood 

 vessels, sympathetic nervous system, connective tissue, the urinary and 

 reproductive apparatus and the walls of the alimentary canal. 



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. 



Dorsal Laminae. As development advances, the true medullary 

 groove deepens, and there arise two longitudinal elevations of the epiblast, 

 the dorsal lamina i one on either side of the groove, which grow u*p, arch 

 over and unite so as to form a closed tube, the primitive central nervous 

 system. 



The Chorda Dorsalis is a cylindrical rod running almost throughout 

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

 blastic cells, and situated immediately beneath the medullary groove. 



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

 of the mesoblast undergo a longitudinal thickening, which develops and 



