THE EMBRYOLOGY OF THE HONEY BEE 207 



the crescents, meet and coalesce in the posterior region of the 

 head, forming here an annular blood lacuna around the oesopha- 

 gus (Fig. 42). On the dorsal side the line of coalescence extends 

 back to and joins the dorsal wall of the heart, so that the latter 

 empties into the annular lacuna, which now constitutes the aorta. 

 This condition obtains only in the extreme posterior region of 

 the head, since the ventral line of coalescence is short, so that the 

 aorta soon comes to open on its ventral side into the general 

 cavity of the head (Fig. 58C). The history of the remainder of 

 the cephalic mesoderm offers little of interest. The mesoderm 

 cells increase by division and arrange themselves gradually into 

 masses corresponding in form to the various muscles of the 

 head, which they are destined to produce. The mesodermal mass 

 which constitutes the principal adductor muscle of the mandible 

 is seen in figure 52 Meso. With the exception therefore of the 

 antennal coelomic sacs, the mesoderm of the entire head, includ- 

 ing the gnathal region, is consumed in the production of muscu- 

 lar tissue. 



The mesoderm of the posterior trunk segments, that is beyond 

 the eleventh trunk segment, has not been studied in detail, owing 

 to the difficulty presented by the flexure of the posterior end of 

 the embryo about the caudal pole of the egg. A flattened mass 

 of mesoderm cells was however observed lying close to the anter- 

 ior side of the proctodaeum (with reference to the axis of the 

 embryo), at the time of appearance of the latter. This mass 

 probably belongs to the terminal or fifteenth trunk segment. At 

 a slightly later stage the proctodaeum is enveloped by mesoderm 

 cells, evidently derived from this mass, and this envelope of 

 mesoderm obviously represents the rudiment of the muscular 

 layer of the proctodaeum. This is in agreement with the observa- 

 tions of Heymons (1895) on Forficula. 



The successive phases of development undergone by the meso- 

 derm, differ to a considerable degree among the representatives 

 of the different orders of insects, nevertheless a fundamental simi- 

 larity exists, and in the development of the mesoderm of the major- 

 ity of insects, subsequent to its establishment as a germ layer, a 

 common type is discernible, more or less modified, according as 

 the insect — speaking generally — is specialized or primitive. In 

 the myriopods, which probably stand as close to the ancestors of 



