130 THE EMBRYOLOGY OF THE HONEY BEE 



ducts of the dermatoblasts. The latter constitutes a single tier 

 of cells which are growing progressively smaller by division, as 

 evidenced by the diminishing size of their nuclei. The form of 

 these cells ranges from cubical, near the mid-line, to long prismatic 

 at the lateral edges of the primitive swellings. 



The neuroblasts, which are frequently to be observed in division 

 (Fig. 49A) are true teloblasts and suffer no diminution in size 

 and are becoming increasingly conspicuous owing to the reduction 

 in size of the dermatogenic cells. The outermost row of neuro- 

 blasts of each side is now seen to differ from the remainder in 

 giving off its daughter cells mesiad instead of centrad. In the 

 honey bee the arrangement of the neuroblasts is not quite so 

 regular as in many other insects, the Orthoptera for example, 

 but as a rule there are from three to five rows of neuroblasts in 

 each lateral cord. The number of rows is greatest in the middle 

 of each segment, as may be seen by comparing figures 48D and 

 49 A. This difference exists in Forficula (Heymons 1895) and 

 Donacia (Hirschler 1909), and is quite probably of frequent 

 occurrence. 



The number of ganglion cells has now increased, but not uni- 

 formly, since some cell nests possess as many as four, others 

 only one. 



At Stage X (Figs. 49B, C, and D) several changes are notice 7 

 able. One of the most evident is the flattening of the ventral 

 surface of the embryo, resulting in the almost complete disap- 

 pearance of the external evidences of both the primitive swellings 

 and neural groove except in the gnathal region (Fig. X). As a 

 result of the flattening of the external surface of the ventral 

 ectoderm the lateral cords are, so to speak, thrust inward and 

 project into the body cavity, especially in the intrasegmental 

 regions. This change in the contour of the ventral ectoderm is 

 probably brought about by the absorption of yolk and the con- 

 sequent diminution of its mass, causing its withdrawal from the 

 ventral ectoderm and 2 lowering of pressure upon the latter from 

 within. The mesial edges of the two halves of the mesoderm 

 now extend up to the lateral boundaries of the lateral cords and 

 about as closely against them (Fig. 49C) that it is frequently not 

 easy to distinguish the neurogenic from the mesodermal tissue, 



