THE EMBRYOLOGY OF THE HONEY BEE 85 



such an extent that in sections they frequently appear as short 

 dark lines (Fig. 30). Comparing the extreme tenuity of the am- 

 nion, when completed, and the superficial extent of each of its 

 component cells as compared with the thickness and slight super- 

 ficial extent of the amnion cells, at earlier stages, particularly 

 those of the head fold, it is apparent that the development of the 

 amnion is due principally, if not exclusively, to a mere extension 

 or spreading out of the original cells present at least as early as 

 Stage IV. This was essentially the view taken by both Biitschli 

 (1870) and Grassi (1884). Neither of these investigators saw 

 any division of the amnion cells, nor has the writer observed 

 them. 



Biitschli (1870), Kowalevski (1871) and Grassi (1884) have 

 described and figured the formation of the amnion in the honey 

 bee. Biitschli's account, based exclusively on observations of 

 fresh eggs, is full and substantially correct, recording, among 

 other details, the covering of the dorsal surface of the egg by 

 the amnion cells. Kowalevski's account is less extended than 

 that of Biitschli, and while correct as regards the topographical 

 relations of amnion and germ band, erroneously describes the 

 amnion as originally composed of two layers, which subsequently 

 fuse to form one. This error was possibly due to Kowalevski's 

 contemporaneous studies on Hydrophilus, in which, as is well 

 known, there are two embryonic membranes. Grassi's account, 

 although based in part on actual sections, adds but little to that 

 of Biitschli. Grassi incorrectly describes the meeting of the ceph- 

 alic and caudal folds as taking place on the ventral surface of 

 the egg midway of its length, whereas it normally takes place at 

 or near the caudal pole of the egg. 



Before entering upon a comparison of the embryonic envelope 

 (amnion) of the bee with the embryonic envelopes of the other 

 pterygote insects, it will not be out of place to recall the manner 

 in which these are formed. Briefly stated, it consists essentially 

 in the elevation, around the embryonic rudiment, of a fold of the 

 extra-embryonic blastoderm, which then extends over the embryo 

 from all sides, its edges finally meeting and fusing (see Kors- 

 chelt and Heider, Fig. 133). Contemporaneous with this fusion 

 is the separation of the two layers composing the fold, so that 



