THE GERMINAL LAYERS. 



359 



The sixth day. By the end 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 embryonal area the upper layer of cells, or Rauber's 

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



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sections. The middle layer of cells, or epiblast (cf. Fig. 141, E), 

 is rather thicker than before, owing 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 



