210 THE EMBRYOLOGY OF THE HONEY BEE 



The formation of the definitive or secondary body cavity is 

 inaugurated by the appearance of the epineural sinus, which is 

 produced mainly by a withdrawal of the yolk from the embryo 

 along the ventral mid-line. As the yolk continues to withdraw 

 from the embryo the epineural sinus becomes extended laterally. 

 Next the dorsal or visceral wall of the coelomic sacs becomes 

 broken through so that their cavities become continuous with the 

 epineural sinus, thus forming the definitive body cavity. 



The development of the mesoderm in the honey bee conforms to 

 the above ideal scheme quite well, except in one rather important 

 particular : the fat body — with the exception of the pericardial fat 

 cells — is formed, not from the somatic wall of the coelomic sacs, 

 but from their visceral wall, or rather that part of it not used up in 

 the production of the enteric muscles. This peculiarity is also 

 shared by the mason bee, Chalicodoma, so that it seems probable 

 that it may be peculiar to the Hymenoptera in general. There 

 appears to be no evident reason for this divergence and it would 

 be futile to speculate concerning it. 



In that region of the insect embryo which is to constitute the 

 definitive head, the segments in this region are naturally much 

 more modified than in the trunk. In Forficula (Heymons 1895) 

 besides a paired mass of mesoderm in front of the stomodaeum, 

 there is a well developed pair of coelomic sacs in the antennal and 

 the three gnathal segments, and in addition a pair of unmistakable, 

 although reduced coelomic sacs in the premandibular segment. In 

 other Orthoptera (see Heymons 1895) a similar condition obtains, 

 except that the coelomic sacs of the premandibular segment are 

 reduced to mere groups of cells. In the embryos of the Coleoptera 

 the accounts of the mesoderm of the head are somewhat con- 

 flicting. Heider (1889) states that in Hydrophilus the coelomic 

 sacs are wanting in the cephalic region, but appear suppressed in 

 the mandibular segment, and their development is delayed in the 

 first maxillary segment. In the most recent account of the devel- 

 opment of a Coleopterous insect (Donacia), Hirschler (1909) 

 finds, in the cephalic region, coelomic sacs only in the intercalary 

 (premandibular) and second maxillary segments. In the Hymen- 

 optera, according to Carriere and Burger's (1897) description of 

 the head mesoderm of Chalicodoma (pp. 392-393) : 



