142 THE EMBRYOLOGY OF THE HONEY BEE 



and posterior commissures are close together, so that it is obvious 

 that the anterior and posterior median gangliomeres, as well as 

 the central gangliomere, are formed from the median cord. 



B. The Brain 



The rudiments of the brain, the procephalic lobes (see p. 100), 

 appear at Stages VI- VII. These together form a heart-shaped 

 expansion of the anterior end of the germ band which at its 

 widest part embraces a trifle over two-thirds of the diameter of 

 the egg. The antero-lateral margin of each lobe is rounded, and 

 the two lobes are separated from one another at the anterior end 

 of the germ band, by a median notch or indentation. Caudad the 

 lobes narrow rather gradually to join the remainder of the germ 

 band. The anterior limit of the procephalic lobes is at this stage 

 slightly ventrad of the cephalic pole of the egg. 



The structure of the procephalic lobes at Stage VII is shown 

 in figures 29, 30, and 31. As these show, the lobes (ProL) are 

 composed of a single thick layer of long prismatic cells which 

 rises slightly above the level of the surrounding blastoderm, 

 (Fig. VII). On comparing the ectoderm of the procephalic 

 lobes with that of the neurogenic area of the trunk it, is at once 

 evident that the ectoderm of the lobes is of much greater thick- 

 ness, particularly in their anterior half (Figs. 30, 31). The 

 nuclei of the cells composing the procephalic lobes are similar 

 to those of the neurogenic area of the trunk ectoderm both in size 

 and clear appearance. 



In cross sections through the posterior region of the proto- 

 cerebral lobes, caudad of the point where the anterior mesenteron 

 rudiment comes to the external surface, there is seen on each 

 side a group of cells (Fig. 31, Hyp) whose nuclei have the small 

 size and dense appearance characteristic of the dermatogenic ecto- 

 derm bordering the neurogenic area of the trunk. By following 

 this group caudad it is found to be actually continuous with this 

 portion of the ectoderm, which may therefore be conceived as 

 sending a tongue-like prolongation forward on each side into the 

 neurogenic portion of the protocerebral lobes. The fate of this 

 group of cells is uncertain, but its position suggests that it may 

 represent the antennal rudiment, which at the following stage 



