THE EMBRYOLOGY OF THE HONEY BEE 151 



XI and XII plainly seen to be marked off into two nearly equal 

 parts by a narrow furrow which runs parallel to the outer margin 

 of the protocerebral lobes. A similar furrow or depression on the 

 opposite side of the epithelial plate combines with the- first to 

 form a constriction which separates the plate into two nearly equal 

 parts, one of which is central, lying next to the second division 

 of the protocerebrum, while the other is marginal or peripheral 

 (Fig. 58A). This half projects laterad nearly free from the 

 mass of cells constituting the adjacent parts of the protocerebrum. 



During Stages XII-XIII the superficial furrow deepens into a 

 cleft, the two halves of the epithelial plate simultaneously bending 

 inward in such a way as to form an invagination of which the 

 cleft is the lumen (Fig. 586). The cells at the bottom of this 

 invagination are considerably deformed, the lateral two-thirds of 

 the epithelial plate being, so to speak, doubled up into a form 

 resembling that of the letter U (Fig. 58C). The peripheral half 

 of the epithelial plate together with the adjacent dematogenic ecto- 

 derm, forms a double fold constituting the lateral boundary of 

 the invagination. During Stage XIII, the two layers of this 

 double fold separate from one another, the outer dermatogenic 

 layer gliding over the optic lobe to join the thin hypodermis 

 formed from the adjacent parts of the brain by delamination 

 (Fig. 58C). This thicker hypodermis overlying the optic lobes 

 now constitutes the optic plate, from which the receptive portion 

 of the compound eye is derived (Fig. 58A, B, and C, OpPl). The 

 optic lobe of each side now appears in either transverse or coronal 

 sections as an oval mass composed of a double layer of large 

 columnar cells, and retains this appearance at least well into the 

 larval period. 



A conception of the superficial extent and direction of the in- 

 vagination concerned in the formation of the optic lobes may be 

 obtained from certain favorable tangential sections of the head. 

 Such a section is represented in figure 58D. The double fold 

 of neurogenic ectoderm forming the optic lobe (OpL) is at Stage 

 XV seem to extend across the head at approximately right angles 

 to the long axis of the protocerebral lobes. These two folds are 

 continuous with one another at both ends of the slit-like lumen 

 of the invagination, so that each optic lobe may be compared to 



