FATE OF THE GERMINAL LAYERS. 



473 



and the whole of the middle germinal layer or mesoblast is 

 derived from the primitive streak by proliferation of the 

 epiblast in this manner. 



Fate of the Germinal Layers. 



The epiblast or covering layer of the blastoderm gives 

 rise to the following organs and tissues of the chick : The 

 epidermis and its appendages (scales, feathers, &c.) ; the 

 nervous system ; the sensory epithelium of the sense-organs ; 

 the lining of the mouth and of the cloaca. 



The hypoblast gives rise to the epithelium of the ali- 

 mentary canal and of the glands that spring from it. 



The mesoblast gives rise to all the connective tissue, 

 vascular, muscular and skeletal structures, as well as to the 

 urinary and reproductive organs. 



The Notochord and Medullary Folds. 



Immediately in front of the primitive streak a less 

 clearly denned opaque area appears towards the end of the 

 first day, which is shown by sections to be a forward con- 

 tinuation of the primitive streak, inseparably fused, however, 



Fia. 84. Transverse section across the body of a Chick Embryo at the 

 twenty-fourth hour of incubation, x 200. 



CH, notochord. E, epiblast. H, hypoblast. M, mesoblast. MA. com- 

 mencing neural ridge. NG, neural grove. NP> neural plate. 



with the hypoblast. This structure, which is called the 

 * head-process,' is destined J to form the notochord or fore- 

 runner of the vertebral column. 



