SECOND PERIOD OF DEVELOPMENT. 65 



conclusion drawn therefrom that the posterior edge 

 of the gastrula-mouth remains unaltered, and, what is 

 most important, the anterior edge suffers alterations 

 during the closing of the gastrula-mouth. The transi- 

 tion from the epiblast to the hypoblast is, that is to 

 say, not similar in all parts of the gastrula-mouth. It 

 is at the posterior edge that the removal of the 

 epiblast from the hypoblast takes place most pro- 

 minently, since there the hypoblast cells are most 

 strikingly distinguished by their size from the epi- 

 blast cells. There we can quite early distinguish two 

 specially large hypoblast cells l situated towards both 

 sides of the central line. These two mark for us the 

 posterior pole of the body, and they serve for us as a 

 starting point, in order to recognise that during the 

 closing of the gastrula-mouth the posterior edge of the 

 latter remains unaltered and corresponds to the later 

 posterior pole of the body. In the remaining periphery 

 of the gastrula-mouth the removal of the epiblast 

 from the hypoblast is less clearly defined, and this 

 gradual transition of the epiblast to the hypoblast is 

 most striking at the anterior edge. This retains the 

 character of a rounded edge, as distinguished from the 

 condition of the hinder edge. 



1 Polar mesoblast cells. F 



