THE EMBRYOLOGY OF THE HONEY BEE 61 



narrow end of the superficial part of the rudiment; it then 

 progresses rapidly both caudad and mesiad at the same time. 

 This process occupies the interval between Stages V and. VII. 

 It is important to note that the mesiad progress of the lateral 

 ectoderm over the anterior mesenteron rudiment and the ap- 

 proximation of the anterior ends of the lateral plates over the 

 middle plate takes place at the same time and goes on at about 

 the same rate. 



The nuclei of the mesenteron cells, spherical in form from the 

 inception of the rudiment, now begins to be more clearly distin- 

 guishable from those of the surrounding tissues, being dis- 

 tinguished not only by their circular outline but also by their 

 somewhat greater size and paleness. These differences serve as 

 useful means of identification during the succeeding stages of 

 embryonic development. 



The form and relations of the anterior mesenteron rudiment to 

 the adjacent parts at Stage VI when the ectoderm is completely 

 formed over the anterior half of the germ band is shown in 

 figure 26B, which represents a median longitudinal section 

 through the anterior end of the germ band. The anterior mes- 

 enteron rudiment is here seen to be thick lenticular in form, its 

 anterior edge projecting out from beneath the anterior margin 

 of the overlying ectoderm. Betweeen the latter and the ecto- 

 derm (Ect) which overlies the mesoderm (Meso) is a rather 

 wide gap representing a rounded break in the continuity of the 

 ectodermal covering of the germ band. This marks the place of 

 origin of the future mouth. Through this gap or orifice, cells of 

 the posterior edge of the anterior mesenteron rudiment are seen 

 to come to the external surface, filling the space with a plug of 

 cells. A similar relation between the anterior mesenteron rudi- 

 ment and the overlying ectoderm occurs in Chalicodoma (Car- 

 riere and Burger 1897, and between both mesenteron rudiments 

 and ectoderm in Gasteroidea (Hirschler 1909^). This plug 

 however is not composed exclusively of the cells of the anterior 

 mesenteron rudiment, since a few mesoderm cells, distinguished 

 by their smaller nuclei, are seen in its posterior portions. Pos- 

 teriorly the anterior mesenteron rudiment is very closely united 

 with the anterior end of the mesoderm, no sharp line of separa- 



