THE EMBRYOLOGY OF THE HONEY BEE 179 



of the tentorium. The second pair constitute the mandibular 

 apodeme. 



Heider (1889), Heymons (1895), Carriere and Burger (1897), 

 Toyama (1900) and Riley (1904) have given complete accounts 

 of the development of the endoskeleton of the head, and with 

 these the account just given is in substantial agreement, and may 

 possibly be considered therefore as representative of the typical 

 mode of development of this structure. 



The tentorium of the honey bee is unmentioned in all previous 

 accounts except that of Grassi (1884). In this are described and 

 figured the two pairs of invaginations which ultimately unite to 

 form the tentorium, but in later stages Grassi lost sight of the 

 anterior pair and consequently failed to grasp their significance, 

 supposing that only the posterior pair were concerned in the 

 formation of the tentorium. 



3. Hypodermis 



The hypodermis is formed of the ectoderm which remains after 

 the subtraction of the various tissues and organs of ectodermal 

 origin. At Stages VII and VIII, the dorsal half of the blastoderm 

 is stripped from the yolk to form the amnion. This portion of 

 the yolk is therefore left entirely bare. A little later at Stage 

 IX a delicate cellular membrane is found covering this previ- 

 ously nude area, except in the cephalic region, which now is 

 covered by the cephalic end of the germ band. This membrane 

 is as thin and delicate as the amnion itself, and is continuous later- 

 ally with the ectoderm, which therefore is presumably responsible 

 for its production. This thin dorsal sheet, for the sake of conven- 

 ience in description, may be termed the extra-embryonic ectoderm, 

 in contradistinction to the embryonic ectoderm, which overlies the 

 mesoderm, and from which are derived the oenocytes, the tracheal 

 system and the ventral nerve cord. The subtraction of this 

 material from the embryonic ectoderm, which takes place during 

 Stages IX-XI Figs. 78 and 79), materially reduces the thickness 

 of the embryonic ectoderm, particularly in its median part, from 

 which the ventral cord is derived. At Stage XI (Fig. 80) the 

 entire embryo is seen to have begun to increase in breadth, this 



