258 THE EMBRYOLOGY OF THE HONEY BEE 



of the brain of all embryos, being especially abundant in the 

 region between the second and third lobes of the protocerebrum. 

 They appear soon after the brain begins to form and are evident 

 up to the time of hatching of the embryo. Their significance is 

 not known. 



The tracheal system is formed from eleven pairs of invagina- 

 tions of the lateral ectoderm. The first of these, situated on the 

 second maxillary segment, by the formation of four diverticula, 

 produces the anterior ends of the main tracheal trunks, including 

 the anterior tracheal commissure or loop, and also the tracheae 

 supplying the head. The ten pairs remaining are situated on the 

 second and third thoracic, and the first eight abdominal segments. 

 These also form each four diverticula. The anterior and pos- 

 terior diverticula become united along each side of the embryo, 

 thus forming the longitudinal tracheal trunks, the ventral diver- 

 ticula fuse with those of the opposite side of the same segments 

 to form the tracheal commissures, the dorsal diverticula form 

 branches supplying the dorsal region of the larva. The openings 

 of the tracheal invaginations remain as the spiracles. 



The tentorium is formed from two pairs of ectodermal in- 

 vaginations. The first pair of these is situated in front of the 

 bases of the mandibles, the second behind the bases of the first 

 maxillae. The invaginations belonging to the first pair grow 

 caudad and mesiad, those belonging to the second pair cephalad 

 and mesiad. All four meet in the median plane to form a struc- 

 ture having the form of an X, extending across the head capsule 

 between the oesophagus and the suboesophageal ganglion. An 

 invagination situated immediately caudad of the base of the rudi- 

 ments of the mandible produces the apodeme for the adductor 

 muscle of the mandible. 



The silk glands appear at an early period as tubular invagi- 

 nations of the ectoderm caudad and mesiad of the bases of the 

 second maxillae. These invaginations rapidly lengthen to form 

 long slender tubes extending the length of the trunk. As the 

 second maxillae approach one another, during the later stages of 

 development, they carry with them the openings of the silk glands, 

 which are thus brought to the ventral mid-line. The second 

 maxillae are then united in such a way that a median unpaired 



