62 THE EMBRYOLOGY OF THE HONEY BEE 



tion being visible, the only distinguishing character being the 

 differing size of the nuclei. 



The source and manner of origin of the anterior mesenteron 

 rudiment remain to be considered. The possibility of its deriva- 

 tion from yolk cells is excluded by the simple fact that yolk 

 cells are never present in this region in sufficient numbers to 

 form such a structure, moreover, the close relation of the rudi- 

 ment with the blastoderm and the similarity of its cells to the 

 cells of the blastoderm make the assumption of any other source 

 than the blastoderm impossible. 



Carriere (1890) and Carriere and Burger (1897) state that 

 the mesenteron rudiments in the mason bee arise from the blasto- 

 derm by proliferation. In the honey bee however during the 

 earlier stages, comprising the period of its most rapid growth, 

 when the greater portion of its mass is formed, cell divisions are 

 virtually absent in that part of the ventral plate from which 

 the mesenteron rudiment arises. This is particularly significant, 

 when it is considered that this period is very brief, extending 

 from Stages IV to V. Since, therefore, the anterior mesoderm 

 rudiment does not arise by proliferation it must be assumed that 

 it arises by immigration. This is strongly suggested by the ap- 

 pearance seen in sections like that represented in figure 25B. At 

 this stage the cells bordering the anterior mesenteron rudiment 

 laterally are plainly seen to intergrade with those of the blasto- 

 derm. This is also true of the section represented in figure 

 25C, but is perhaps less evident. 



Two phenomena coincident with the formation of the anterior 

 mesenteron rudiment take on a special significance when con- 

 sidered in association with the facts just mentioned. These are: 

 first, that the appearance of the anterior mesenteron rudiment 

 and the depression of the middle plate are contemporaneous ; 

 second, that the two lateral halves of the ventral plate bounding 

 the mesenteron rudiment appear to move mesiad during the 

 period when the lateral plates are coming together to cover the 

 middle plate, or, to express it in a different way, the lateral 

 halves of the ventral plate bounding the anterior mesenteron 

 rudiment behave toward the latter much as the lateral plates 

 behave toward the middle plate. Figures 24A and B illustrate 

 this point. Figure 24A is drawn from a section through the 



