224 THE EMBRYOLOGY OF THE HONEY BEE 



at about the point of junction of the two rudiments. Near the 

 posterior end of the yolk the epithelium is especially thin, as 

 shown in figure 88B, PMR. At Stage X (Fig. 79 Mint) the 

 posterior end of the yolk is covered by the future epithelium of 

 the mid-intestine which now extends ventrad on each side to meet 

 the splanchnic layer of the mesoderm. During the next three 

 stages (Figs. 80, Si and 82) the epithelium of the mid-intes- 

 tine extends steadily ventrad on each side, until, at Stage XIII- 

 XIV, the two edges meet and unite on the ventral mid-line (Fig. 

 82) thus completing the formation of mesenteron. The union ap- 

 pears to be virtually simultaneous throughout its extent. Mean- 

 while the rudiments of the fore- and hind-intestines, the stomo- 

 daeum and proctodaeum, are also developing. The stomodaeum 

 puts in its appearance at Stage VIII in the form of a cup-shaped 

 depression located at the cephalic pole of the egg. The floor of this 

 invagination is formed by cells of the anterior mesenteron rudi- 

 ment, as shown in figure 87A. Between Stages VIII and IX the 

 depression deepens rapidly, becoming funnel-shaped, while at the 

 same time the yolk retreats from the anterior end of the egg, its 

 withdrawal keeping pace with the lengthening of the stomodaeum. 

 The latter thus soon becomes a short tube, composed of a single 

 layer of prismatic ectodermal cells, closed at its inner end by the 

 cells of the anterior mesenteron rudiment (Figs. 52 and 60, Stow). 

 During the earlier phases of the development of the stomodaeum, 

 its cavity narrows gradually from its outer to its inner end, and 

 the cells of the mesenteron closing the latter form a wall almost 

 as thick as that of the stomodaeum itself, containing several 

 nuclei (Fig. 60). Towards Stage XIV the inner end of the 

 stomodaeum widens out, acquiring a flaring form, and the stomo- 

 daeal wall becomes much thinner just anterior to its junction with 

 the mesenteron (SgA). Careful examination of preparations of 

 this stage shows that this condition is associated with the forma- 

 tion of a double fold of the stomodaeal wall preparatory to the 

 invagination or intussusception of the inner end of the stomo- 

 daeum into the cavity of the mid-intestine. Correlated with these 

 changes is a thinning of the cellular diaphragm-like wall closing 

 the inner end of the stomodaeum, the central portion of this wall 

 becoming reduced to a thin membrane, in whch now no nuclei are 



