60 THE EMBRYOLOGY OF THE HONEY BEE 



Am 



AMR 



' "til 



ill 



A 



B 



FIG. 26. Median sagittal sections through the anterior mesenteron rudi- 

 ments of two eggs, illustrating the development of the rudiment and its 

 relations to the adjacent ectoderm (Bet) and 'mesoderm (Meso). The 

 formation of the amnion (Am) in this region is also s ; hown. A is from an 

 egg of Stage IV- V, B from an egg of Stage VI, x 290. 



of the anterior mesenteron rudiment, thereby gradually restrict- 

 ing its superficial area, and leading finally to the complete cover- 

 ing of its external surface by ectoderm, with the exception of a 

 circular area near its posterior border (Fig. 26B). The precise 

 nature of this process is uncertain. Carriere (1890, 1897) has 

 stated that in the mason bee it consists in a centripetal differen- 

 tiation of the superficial cells of the rudiment into prismatic 

 ectoderm cells, but in the honey bee there are certain concomit- 

 ant phenomena not evident in the mason bee, which make it 

 seem possible that the covering of the anterior mesenteron rudi- 

 ment is brought about by a simultaneous mesiad movement of 

 the two halves of the ventral plate separated by the rudiment. 

 This covering by the ectoderm is completed first at the anterior 



