THE EMBRYOLOGY OF THE HONEY BEE 59 



its apex is near the anterior limits of the ventral plate, while 

 its base joins the anterior end of the middle plate. 



In the next figure, 25B, taken from an egg intermediate be- 

 tween Stages IV and V the mesenteron cells are seen to have 

 increased greatly in number, now forming collectively a low 

 rounded swelling on the inner side of the blastoderm in the 

 mid-line. It is at this stage that the anterior mesenteron rudi- 

 ment first becomes visible from the exterior. The arrangement 

 of the component cells is still loose, numerous interstices re- 

 maining between them. In form the cells are, generally speak- 

 ing, ovoid or rounded, although somewhat irregular. On one 

 side of the nucleus a vacuole is frequently to be seen, as in the 

 adjacent blastoderm cells, which they closely resemble in all 

 respects except that of form. As the cells of the anterior 

 mesenteron rudiment continue to increase rapidly in number 

 (Fig. 25C), the rudiment increases correspondingly in thickness 

 and breadth, so that it spreads out laterally. At the same time, 

 its cells become compactly arranged and polyhedral in form, 

 due to mutual pressure doubtless caused by the necessary dis- 

 placement of the underlying yolk. Figure 26A represents a 

 longitudinal section through the anterior mesenteron rudiment at 

 about this period (Stage IV-V). The rudiment extends cep- 

 halad to within three or four cells of the anterior end of the 

 ventral plate, where it ends abruptly, its cephalic face perpen- 

 dicular to the surface of the blastoderm, leaving in front of the 

 rudiment a cavity bounded on its inner side by the yolk, on its 

 outer by the amnion (Am) and by the short stretch of unmodi- 

 fied blastoderm cephalad of the mesenteron rudiment. Caudad 

 the latter diminishes gradually in thickness to join the middle 

 plate (MP). 



Figure 25D shows the rudiment after the lateral folds have 

 commenced to unite (Stage V-VI) being drawn from the same 

 preparation as figure 21. The anterior mesenteron rudiment 

 has now increased considerably in the number of its cells and in 

 its lateral extent, coming into close contact with the inner sur- 

 face of the lateral blastoderm on both sides of the mid-line. In 

 addition to its increase in size another change is becoming evi- 

 dent, which has the appearance of an encroachment of the 

 lateral unmodified blastoderm (ectoderm) on the external layers 



