THE EMBRYOLOGY OF THE HONEY BEE 173 



-Mint 



Trlnv' 



FIG. 67. Transverse section through the anterior half of the second 

 maxillary segment of an embryo, Stage X, showing one of the pair of 

 tracheal invaginations (Trlnv} and the mesoderm (Meso) belonging to 

 this segment, x 387. 



clearly shown in figure Xa, but not labeled. At the beginning of 

 Stage X the opening of the tracheal sac to the exterior has already 

 become contracted, and by the end of this stage or the beginning 

 of the next (XI) is completely closed, leaving behind no trace of 

 its existence. At the same time the ectodermal folds also disap- 

 pear, their disappearance being in part at least due to the shrink- 

 age and withdrawal of the yolk from this region. 



The further history of the three remaining diverticula of each 

 of the tracheal sacs remains to be considered. All of these elon- 

 gate rapidly, assuming a tubular form. The dorsal diverticulum, 

 which is the largest of the three (Fig. 68, ATraL), continues to 

 extend dorsad, taking a slightly cephalad course, skirting the 

 posterior margin of the cerebral lobes, along the line of junction 

 of the head and trunk. At Stage XIII the distal ends of these 

 branches reach the dorsal mid-line, when they unite to form the 

 anterior tracheal loop or commissure (Figs. 63 and 64, ATraL). 

 Of the two remaining diverticula the ventralmost forms the larger 



