THE EMBRYOLOGY OF THE HONEY BEE 47 



pressed, especially at its lateral margins, as the corresponding 

 deformation of the yolk shows, while the lateral plates are not 

 perceptibly lifted up from the yolk. Figures 20, A and B, il- 

 lustrate the stage next following. Figure 20A is a cross section 

 through the egg near the point when the lateral folds are closest 

 together; figure 20B is near their posterior ends. The middle 

 plate has at this stage changed somewhat in structure in its 

 anterior portions. The marginal cells, formerly like the remain- 

 der of the ventral blastoderm, slender and prismatic, with their 

 long axes directed radially, have turned laterad under the lateral 

 plates, becoming irregularly polyhedral in form. Corresponding 

 with this change the middle plate becomes much broader and 

 thinner, especially at its edges. Figure 20B illustrates the for- 

 mation of the middle plate in the posterior half of the egg. It 

 is evident at a glance that the process is identical with that in 

 the more anterior regions of the egg. There is, however, this 

 difference : that before the middle plate shows any signs of 

 separation from the lateral plates it is distinguishable by being 

 thinner than the lateral portions of the blastoderm, and its cells 

 are correspondingly shortened and widened. This is evident in 

 figure 20B. It is also evident that the initial width of the middle 

 plate is greater in the posterior than in the anterior regions of 

 the egg. This is probably correlated with the thinning of the 

 incipient ventral plate which brings about a corresponding ex- 

 tension of its surface. 



The final stages in the formation of the mesoderm are shown 

 in figure 21, which is a transverse section through the middle of 

 an egg a trifle younger than Stage VI, showing the lateral folds 

 about to unite in the mid-line, the middle plate being now vir- 

 tually completely covered. Its width is — at this point — fully 

 equivalent to one quarter of the circumference of the egg; near 

 the ventral mid-line its thickness is approximately that of the 

 overlying lateral plates (ectoderm), it thins out rapidly to one- 

 half of this thickness at the lateral margins. Here it is composed 

 of but one cell layer ; near the ventral mid-line it can be distinctly 

 seen to be made up of two layers. This arrangement of the 

 mesoderm cells into two layers is more evident in sagittal sec- 

 tions passing somewhat laterad of the median plane (Fig. 23B). 

 It is also more evident in the posterior than in the anterior half 



