THE EMBRYOLOGY OF THE HONEY BEE 63 



anterior end of an egg at Stage IV ; figure 24B is drawn from 

 a section through the same region at a stage about half way be- 

 tween Stages IV and V. The mesiad movement of the two 

 lateral halves of the ventral plate in this region during the inter- 

 vening period becomes at once evident, since the area covered 

 by the dorsal strip and the amnion-forming cells has become 

 greatly extended transversely, accompanied by a flattening of 

 the amnion-forming cells. The possibility that this movement 

 is only an apparent one, and caused by a contraction of the 

 lateral blastoderm is excluded, since the blastoderm cells are 

 not perceptibly lengthened in the section taken at the later 

 stage, as would be the case if an actual contraction had taken 

 place. 



It appears then, in view of the facts outlined above, that the 

 anterior mesenteron rudiment is derived from the cells of the 

 ventral blastoderm by immigration, and that this rudiment is 

 continuous with, and comparable to, the ventral plate, in so far 

 as both are formed contemporaneously from the middle section 

 of the ventral plate, accompanied by a mesiad movement of its 

 lateral sections. Moreover, the anterior mesenteron rudiment 

 and the ventral plate are directly continuous with one another, 

 the chief differences between them being that the former arises 

 as a heap of cells not sharply marked off from the remainder 

 of the blastoderm, while the latter arises as a solid flat section of 

 the blastoderm, discontinuous with the lateral blastoderm and 

 separated from it by a sharp break. 



Cell division, although apparently playing no part in the forma- 

 tion of the anterior mesenteron rudiment during its earlier 

 stages, steps in during the later stages, when mitotic figures are 

 frequently seen, being most abundant in the posterior part of the 

 rudiment. Two mitotic figures are evident in this region in 

 figure 26B. 



The manner in which the ectoderm finally closes over the an- 

 terior mesenteron rudiment is, as before mentioned (p. 60), ob- 

 scure, and repeated efforts finally to decide this question were 

 without definite result, nevertheless, since it is certain that the 

 lateral blastoderm bounding the anterior mesenteron rudiment 

 moves mesiad during the early stages of the rudiment it seems 

 possible that the final covering of the external surface is brought 



