98 COMPARATIVE PHYSIOLOGY. 



tient observation, it is now settled that all the parts of the most 

 complicated organism arise from the three-layered blastoderm 

 previously figured ; every part may be traced back as arising in 

 one or other of these layers of cells — ^the epiblast, mesoblast, or 

 hypoblast. It frequently happens that an organ is made up of 

 cells derived from more than one layer. Structures may, ac- 

 cordingly, be classified as epiblastic, mesoblastic, or hypoblastic ; 

 for, when two strata of cells unite in the formation of any part, 

 one is always of subordinate importance to the other : thus the 

 digestive organs are made up of mesoblast as well as hypo- 

 blast, but the latter constitutes the essential secreting cell mech- 

 anism. As already indicated, the embryonic membranes are 

 also derived from the same source. 



The epiblast gives rise to the skin and its appendages (hair, 

 nails, feathers, etc.), the whole of the nervous system, and the 

 chief parts of the organs of special sense. 



The mesoblast originates the vascular system, the skeleton, 

 all forms of connective tissue including the framework of 

 glands, the muscles, and the epithelial (endothelial) structures 

 covering serous membranes. 



The hypoblast furnishes the secreting cells of the digestive 

 tract and its appendages — as the liver and pancreas— the lining 

 epithelium of the lungs, and the cells of the secreting mucous 

 membranes of their framework of bronchial tubes. 



It is difiicult to overrate the importance of these morpholog- 

 ical generalizations for the physiologist ; for, once the origin of 

 an organ is known, its function and physiological relations gen- 

 erally may be predicted with considerable certainty. We shall 



Fia. 100. — Transverse section tlirongh the mednllary groove and halt the hlastoderm 

 of a chick of eighteen hoars (Foster and Balfour). E, epiblast; M, mesoblast; 

 R, hypoblast; m/, mednllary fold; mg, medullary groove; ch, notochord. 



endeavor to make this prominent in the future chapters of this 

 work. 



Being prepared with these generalizations, we continue our 

 study of the development of the bird's embryo. Before the end 



