THE EMBRYOLOGY OF THE HONEY BEE 229 



of invaginations lying closest to the caudal pole of the egg the 

 future ventral pair of Malpighian tubules are deeper than the 

 anterior pair, and this difference is evident up to a late stage. 

 At Stage X the ectodermal area included between the grooves 

 described above becomes depressed, carrying with it the rudi- 

 ments of the Malpighian tubules (Fig. 91, Mai). This area 



Proc 



,-Meso 



FIG. 91. Transverse section through the posterior end of an embryo, 

 Stage X, intersecting the proctodaeal invagination (Proc). The posterior 

 (ventral) pair of Malpighian tubules (Mai) are also shown. 



therefore constitutes the floor of the proctodaeum. The open- 

 ing of the latter is at first on the dorsal side of the egg, on account 

 of the caudal flexure of the embryo, but with the gradual 

 straightening of the embryo the external opening of the procto- 

 daeum becomes terminal with respect to the egg. This takes 

 place during the next stage (XI). From the time of its inception 

 the proctodaeum elongates slowly and steadily, the yolk retreating 

 from the posterior end at a corresponding rate. The inner end 

 of the proctodaeum comes into contact with the epithelium of the 

 mesenteron from the first, but the relation does not become more 

 intimate than that of mere contact. The growth of the Mal- 

 pighian tubules is extremely rapid ; they push forward along the 

 ventro-lateral wall of the mid-intestine as slender, nearly straight 

 tubes, until their anterior ends reach the neighborhood of the 

 sixth trunk segment. Here their cephalad extension seems in 

 some way hindered, and their further elongation causes them to 

 be thrown into a series of curves and loops (Fig. XV, Mai). 



