156 THE EMBRYOLOGY OF THE HONEY BEE 



together the two halves of the tritocerebrum, and is accompanied 

 by a few small cells, apparently ganglion cells. This band of 

 fibres is formed at Stage X-XI by processes of the ganglion cells 

 of the tritocerebral lobes (Fig. 55, SoeCom). These fibres lie in 

 a minute channel at the bottom of the reentrant angle formed by 

 the junction of the hinder wall of the oesophagus and the ventral 

 ectoderm and lie immediately above the extreme anterior end of 

 the median cord (Fig. 59A, SoeCom). During Stage XI after 

 the suboesophageal ganglion separates from the ventral hypoder- 

 mis the suboesophageal commissure also leaves the ventral body 

 wall, although remaining in contact with that of the oesophagus. 

 During the stages succeeding, the hypodermis directly caudad of 

 the mouth grows out to form the lower lip or hypopharynx, in- 

 creasing the space below the oesophagus and consequently the 

 distance of the circumoesophageal commissure from the ventral 

 surface. Up to Stage X the nerve fibres of the suboesophageal 

 commissure and the anterior end of the median cord, now an 

 integral part of the suboesophageal ganglion, remain in close ap- 

 position (Fig. 59A). At Stage XIII-XIV (Fig 45) these become 

 separated by a slight cleft traversed by a few muscle fibres (dila- 

 tors of the pharynx) which pass from the median part of the 

 tentorium to the posterior or lower wall of the pharynx. 



In view of the close apposition of the fibres of the circumoeso- 

 phageal commissure and the anterior end of the median cord, and 

 of the serial homology of the commissures of the ventral cord 

 with those of the brain, it is not impossible that the cells accom- 

 panying the fibres of the suboesophageal commissure are derived 

 from the ventral cord. This point could not however be definitely 

 determined, in spite of repeated efforts. While it may be true 

 that these cells are actually derived from the median cord, they 

 have rather the appearance of having migrated mesiad from the 

 two lobes of the tritocerebrum. 



Summary 



In summing up the foregoing observations on the development 

 of the brain of the honey bee, with reference to their bearing on 

 the development of the brain of other insects, it may be said, 

 first, that in a broad sense they confirm the results of Viallanes, 



