THE EMBRYOLOGY OF THE HONEY BEE 



81 



on the right hand side of the section, in place of mesoderm, two 

 cells belonging to the anterior mesenteron rudiment are seen 

 (AMR). 

 Figure 32 shows a section taken through the mid-region of the 



"--Meso 



Ect 



FIG. 32. Transverse section through the future thoracic region of an 

 embryo, Stage VII-VIII, showing the form and relations of the ectoderm 

 (Ect) and mesoderm (Meso), x 243. 



trunk, and is representative of the condition existing through- 

 out its extent, including the future thorax and abdomen. The 

 ectoderm covers somewhat less than the ventral half of the egg, 

 and is composed of a single layer of closely packed columnar 

 cells. Within the ectoderm is the mesoderm (Meso), whose 

 lateral extent is somewhat less than that of the ectoderm. The 

 mesoderm, near the mid-line, is somewhat thinner than the ec- 

 toderm, but towards its lateral margins, which are rounded, its 

 thickness approaches that of the ectoderm. It is clearly com- 

 posed of two layers, continuous with one another at the lateral 

 margins. Near the mid-line in its thinner portions the cells 

 composing the two layers are flat and the line of separation be- 

 tween the layers is somewhat indistinct. Toward the lateral 

 margins of the mesoderm, however, the cells composing the two 

 layers become columnar, and the line of separation between the 

 layers is sharp. This line of separation represents the cavity of 

 the mesodermal somites. 



A section through the caudal end of an embryo of Stage VII 

 has already been represented in figure 28, and described in the 

 part of this section relating to the posterior mesenteron rudiment. 



