THE GEEMINAL LAYERS. 



359 



The sixth day. By the ejid of the sixth day, the blastodermic 

 vesicle has a diameter of 3 to 3'5 mm., and the embryonal area, 

 which is still approximately circular in outline, measures 0-75 

 mm. across. 



In the embiyonal area the upper layer of cells, or Rauber's 

 layer, is thinner than before, and very diflBcult to recognise in 







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sections. The middle layer of cells, or epiblast (c/. Fig. 141, e), 

 is rather thicker than before, owiag to a change in the shape of 

 the individual cells, which are now columnar in place of being 

 cubical. The lower layer, or hypoblast, consists, as before, of a 



