THE EMBRYOLOGY OF THE HONEY BEE 127 



A. The Ventral Cord 



At Stage VII, when the formation of the germ layers is vir- 

 tually completed the ectoderm of the germ band caudad of the 

 procephalic lobes forms a uniform and rather thick layer com- 

 posed of prismatic cells and covers the ventral half of the egg 

 (Figs. 32, 48A). The cells of the ectoderm are not, however, 

 all alike, those comprising its middle third having nuclei consid- 

 erably larger and somewhat clearer than those of its lateral por- 

 tions ; this median area may be termed the neurogenic area, since 

 it includes the cells from which the ventrad cord will arise. In 

 the mid-ventral line there is a narrow strip, three to five cells 

 wide, which is quite well denned in the anterior region of the 

 germ band, less so in the posterior regions, the median cord (Fig. 

 48A, MC). With this exception the cells comprised within the 

 neurogenic area are at this stage precisely similar as regards form, 

 size, or staining reaction. 



At the next Stage, VIII (Figs. 46B and C) several notable 

 changes are evident. The ectoderm is greatly reduced in thick- 

 ness laterad of the neurogenic area and also along the ventral 

 mid-line. The neurogenic area therefore comprises two longi- 

 tudinal thickenings, the primitive swellings (Fig. 48B, PriSiv) 

 which are separated by a median furrow, the neural groove 

 (NIG.) The primitive swellings do not owe their formation to 

 cell proliferation, but to changes in the form of the cells of differ- 

 ent areas, the primitive swellings alone preserving the thickness 

 of the ectoderm and the primatic form of its cells existing at 

 Stage VII. 



The primitive swellings, also shown in figures VIII, IX and X, 

 are highest in the gnathal region, and diminish rapidly in height 

 caudad, becoming scarcely distinguishable in the abdominal re- 

 gion. Moreover they are broader and slightly higher in the intra- 

 segmental than in the intersegmental regions, as a comparison of 

 figures 48B and C, and 48D and 49A will show, being thus divided 

 into a series of segmental ganglionic swellings corresponding to 

 the future ganglia. 



The median cord can now be readily identified ; its cells form 

 the floor of the neural groove. At this stage the median cord is 

 much broader and thinner than either at the stage preceding, or 



