THE GERMINAL LAYERS. 



359 



The sixth day. By the end of the sixth clay, 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 



U^- 



»jtions. The middle layer of cells, or epiblast (cf. Pig. 141, e), 

 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 



