228 THE EMBRYOLOGY OF THE HONEY BEE 



completely disappeared. The posterior mesenteron rudiment, 

 gives perhaps the best indication of the former existence of paired 

 bands in the incomplete bilateral subdivision of its thicker basal 

 portion at Stage VIII . 



The rudiments of the Malpighian tubules appear at Stage 

 VIII, preceding, as Grassi (1884) discovered, the formation of 

 the proctodaeum, which does not appear until Stage IX. The 

 rudiments of the Malpighian tubules therefore for a short time 

 open directly on the external surface of the embryo. A similar 

 relation obtains in Microgaster (Kulagin 1892), Leptinotarsa 

 (Doryphora) {Wheeler 1889), Chalicodoma (Carriere 1890, 

 Carriere and Burger 1897), and Gasteroidea (Hirschler 1909). 

 It is only in Apis and Chalicodoma however that the Malpighian 

 tubules are actually formed before the proctodaeum ; in the others 

 they appear on the external surface of the ectoderm around the 

 already formed proctodaeum. In the honey bee the Malpighian 

 tubules appear as four equidistant pit-like depressions at the 

 extreme posterior end of the germ band, on the dorsal surface of 

 the egg. According to Grassi's account the depressions are at 

 first entirely distinct from one another, but subsequently the two 

 lying on the same side of the mid-line merge with one another, 

 a pair of longitudinal grooves being thus formed. Examination 

 of preparations of embryos of the proper stage (VIII) failed to 

 show the invaginations as distinct from one another; in all cases 

 in which they were sufficiently well defined to be recognizable 

 they had coalesced on each side of the median line to form a pair 

 of slightly curved grooves, whose concavities faced one another 

 (Fig. 90). Examination of sections shows also that the pair 



_vMal 



Fig. 90. Dorsal surface of posterior end of egg, Stage IX, showing the 

 escentic grooves from which the Malpighian tubules are formed. 



