THE EMBRYOLOGY OF THE HONEY BEE 211 



"In an embryo, in which the mesoderm tubes are present we can 

 generally follow the mesoderm forward into the antennal segment. 

 We can determine, that it fills the antennae and all the jaw rudi- 

 ments, inclusive of the premandibular rudiment, forming their 

 cores. Its greatest development is attained by the mesoderm 

 in the sides of the head segments, where it is represented by sev- 

 eral cell layers, it is wanting in the middle or only forms, as in 

 the antennal and premandibular segments, a small bridge con- 

 sisting of a single sheet of scattered cells. 



Possibly even before the appearance of the cavities which rep- 

 resent the beginnings of the primitive body cavities, a cleft 

 becomes evident in the head region of the germ band, between 

 ectoderm and yolk (precisely between the rudiments of the ventral 

 ganglionic chain and the yolk), in this cleft the mesoderm cells 

 are much scattered. This space later is continuous with the 

 epineural sinus, and the mesoderm cells scattered, that is floating 

 in it, are to be regarded as blood cells. It later extends greatly, 

 although the mesoderm contained within it can scarcely be said to 

 increase and in general remains limited to the appendages and 

 their bases. Therefore in the head section of Chalicodoma there 

 is no formation of coelom. The antennal segment however is an 

 exception. 



A little later coelomic cavities appear in the antennal segment. 

 This condition is virtually paralleled in the honey bee. 



The fate of the head mesoderm as gleaned from the various 

 accounts, is briefly as follows: In Forficula (Heymons 1895) 

 the mesodermal mass anterior to the antennal segment forms the 

 muscular coat of the fore-intestine. The antennal coelomic sacs 

 become greatly extended, particularly in a longitudinal direction, 

 while their mesial walls become thickened, and finally join in 

 such a way as to form a tube, the aorta, continuous with the heart 

 at its caudal end. The mesoderm in the remaining segments is 

 consumed in the production of muscles, except in the premandi- 

 bular segment. Here the mesoderm forms the "suboesophageal 

 body" described by Wheeler (1893). In the Coleoptera as repre- 

 sented by Donacia (Hirschler 1909) the muscular coat of the 

 fore-intestine is apparently formed by the mesoderm situated in 

 the anterior head region, judging by the figures. The aorta is 

 formed by coelomic sacs in much the same way as in Forficula, 

 except that Hirschler confidently asserts that these belong not to 

 the antennal, but to the premandibular segment. In Chalicodoma 

 the course of events is much like that in Forficula, except that no 



