66 



THE EMBRYOLOGY OF THE HONEY BEE 



rudiment is seen to have increased greatly in thickness. It now 

 forms a discoid mass lying on the ventral surface of the egg, 

 its posterior boundary lying almost precisely at the caudal pole. 

 Its extent is approximately the same as that of the anterior 

 mesenteron rudiment. As the illustration shows the posterior 

 mesenteron rudiment is now composed of long irregularly bent 

 and curved fusiform cells, some of which at least extend through- 

 out the entire thickness of the layer. At the next stage, Stage 

 VII (Fig. 270), four changes are seen to have taken place, (i) 

 The rudiment, by a lengthening of the germ band, has been 

 shifted around the caudal pole of the egg to the dorsal surface. 

 (2) It is no longer composed of long fusiform cells, but of 

 polyhedral cells precisely like those of the anterior mesenteron 

 rudiment. (3) The ectoderm now covers its anterior half. (4) 

 It has increased greatly in thickness, which approximates one- 

 half its diameter. The question as to the manner in which the 

 posterior mesenteron rudiment becomes covered by ectoderm is 

 even more difficult to answer than in the case of its counterpart 

 at the anterior end. That this covering is brought about by ex- 

 tension of the ectoderm at the expense of the superficial cells 

 of the rudiment is strongly suggested by the section represented 

 in figure 270. It is certain at least that the extension takes 

 place in an antero-posterior direction with regard to the embryo 

 itself, and that the ectodermal covering of the rudiment is 

 absolutely continuous with that of the lateral plates, since in 

 one series (that from which Fig. 28 was drawn) the cleft still 

 separating the lateral plates can be followed around the caudal 

 end of the egg to the posterior limit of the ectoderm. Figure 28 

 represents a transverse section through the posterior end of an 

 embryo of Stage VII, and intersects the posterior mesenteron 

 rudiment about midway of its length. On the ventral side are 

 seen the lateral plates (LP) still separated by a narrow cleft, and 

 lying above them, the middle plate (MP), or mesoderm. The 

 dorsal half of the section is occupied by the massive posterior 

 mesenteron rudiment (PMR), connected on each side with the 

 lateral plates by a thin sheet of cells, the amnion. This figure 

 illustrates an important difference between the anterior and 

 posterior mesenteron rudiments. While the former is produced 

 only by a relatively narrow median strip of blastoderm, the lat- 



