64 THE EMBRYOLOGY OF THE HONEY BEE 



about in this manner. Sections like that represented in figure 

 25D lend color to this supposition, since here the mesial edges 

 of the lateral blastoderm are marked off from the mesenteron 

 cells with a considerable degree of sharpness. 



The posterior mesenteron rudiment is much more difficult to 

 study than the anterior rudiment, for two reasons ; first, being 

 formed near the posterior pole, in the course of its development 

 the lengthening of the germ band carries the rudiment over the 

 rounded posterior end towards the dorsal side, so that sections 

 normal to the surface of the germ band at this point are seldom 

 obtained, and second, the posterior end of the egg, being the 

 one by which it is attached to the floor of the cell of the comb, 

 is frequently damaged. For the first of these reasons, longitudi- 

 nal sections are more informing and more useful than transverse 

 sections, which are commonly cut at right angles to the long 

 axis of the egg, and therefore only rarely intersect the posterior 

 mesenteron rudiment at right angles to its own long axis. As a 

 matter of fact, scarcely any satisfactory transverse sections 

 through this rudiment were obtained. 



The development of the posterior mesenteron rudiment is 

 fundamentally identical with that of the anterior mesenteron rudi- 

 ment, but differs greatly in details. Unlike the anterior mesente- 

 ron rudiment, the posterior mesenteron rudiment is ordinarily not 

 evident on preparations of entire eggs, except during the conclud- 

 ing stages of its development, when it appears as a deeply stained 

 discoid mass at or dorsal to the caudal pole. Prior to Stage V 

 the ventral plate extends over the caudal pole to the dorsal sur- 

 face as a single layer of low cells, rather irregular in form, and 

 rounded on their external surface. This layer is frequently in- 

 terrupted by short gaps which leave bare the yolk beneath. At 

 Stage V the first indication of the posterior mesenteron rudiment 

 becomes visible as a slightly thickened area of the blastoderm 

 just cephalad of the caudal pole, on the ventral surface (Fig. 

 27A, PMR). The cells composing this thickening are now long 

 prismatic in form, instead of low and rounded as before. The 

 posterior portion of the ventral plate has apparently meanwhile 

 contracted, at least in a longitudinal direction, since it now ex- 

 tends only a short distance dorsad to the caudal pole. At the 

 stage following, Stage VI (Fig. 27B), the posterior mesenteron 



