THE EMBRYOLOGY OF THE HONEY BEE 155 



prior to hatching suggests that the folding of the optic lobe in the 

 bee — and in the Coleoptera — may correspond to the subdivision 

 of the optic lobe in the forms first mentioned into the ganglionic 

 layer and the external medullary mass. This is however only 

 a surmise. 



Both the supra- and suboesophageal commissures have been 

 compared by writers on the insect brain to the commissures of 

 the ventral cord, and are supposed to be formed in the same way, 

 that is, by a median ingrowth of the ectoderm comparable to the 

 median cord. Heymons has observed this in both commissures 

 in Forficala. In the honey bee the cellular portion of the supra- 

 oesophageal commissure is formed from a median thickening of 

 the ectoderm of the head. Figure 59B, taken from a transverse 



Hyp DegCI 



Fig. 59. A,median sagittal section through the mouth of an embryo, 

 Stage X, showing the suboesophageal commissure (SoeCom). B, part of a 

 transverse section of an embryo, Stage XIII-XIV, showing the formation 

 of the supraoesophageal commissure, x 600. 



section of Stage XIII-XIV sufficiently illustrates this point. As 

 in the commissures of the ventral cord it is the cellular portion 

 principally which is furnished by the median ingrowth, the bridge 

 of nerve fibres arising principally from the ganglion cells of the 

 paired ganglia lying on each side of the mid-line. 



The origin of the suboesophageal commissure, or even a part 

 of it, from the ectoderm of the mid-line, is much less clear. As 

 figure 41, Stage XV, shows, the commissure (SoeCom) consists 

 principally of an extremely thin band of nerve fibres connecting 



