156 EICHAED ASSHETON. 



pointed out, has been stretched to a very high degree of 

 tenuity. 



From the middle of the sixth day the embryonic disc area 

 becomes an area of increased activity. 



We have at this moment, therefore, an outer extremely 

 attenuated membrane under high tension due to the hydro- 

 static pressure within. Each " cell " of this membrane repre- 

 sents an individual minute centre of protoplasmic activity. 

 These are so drawn out that a few will extend over a con- 

 siderable area. 



Closely pressed against these is a layer of rounded or but 

 slightly compressed cells, the inner epiblastic layer. Each cell 

 of this layer also represents an individual minute centre of 

 protoplasmic activity, and it will be seen that in any given 

 area of the embryonic disc for one of these little centres of 

 activity of the outer layer (Rauber's) there are three, four, or 

 five of the little centres of activity in the inner epiblastic 

 layer. 



The hypoblast, although it will tend to press the inner 

 layer of epiblast cells firmly against the Rauber layer, need 

 not necessarily be affected by what I am going to describe, 

 because at no time does the hypoblast, after its initial separa- 

 tion from the epiblast, appear to be at all firmly attached to 

 the epiblast. Note the readiness in which it stands away 

 from the epiblast in sections (v. all my figures, and those of 

 KoUiker). 



To return to the two epiblast layers. It is clear now that 

 if the two epiblast layers of the embryonic disc acquire an in- 

 creased activity, then if for any given area the inner epiblast 

 contains, say, three times more cells than the outer epiblast, 

 then the inner epiblast will increase its bulk, roughly speaking, 

 three times more rapidly than the outer epiblast. 



This might produce several effects. It might produce a 

 heaping up of cells ; it might produce an arching inwards of the 

 inner layer of epiblast ; it might produce an extension of the 

 inner epiblast by sliding over the outer epiblast (or ratlier be- 

 tween the outer epiblast and hypoblast) ; or it might cause the 



