THE EMBRYOLOGY OF THE HONEY BEE 205 



crescentic in outline. The formation of the heart is obviously 

 completed by the fusion of the two cardioblast strands (see 

 Fig. 76). 



The two rudiments of the dorsal diaphragm {DDph) are seen 

 to be formed from a single layer of more or less rounded cells, 

 originally of uniform size. These, at Stages XIII and XIV 

 (Figs. 83 A and B, DDph) are seen to have lost this uniformity, 

 cells of two sizes being now present, some of them having nuclei 

 whose size approaches that of the fat cells, while others have 

 nuclei only about half as large. The larger cells moreover have 

 a rounded contour, the smaller on the contrary are thin and flat. 

 These are to be considered as constituting, in a strict sense, the 

 diaphragm, while the larger rounded cells are the paracardial cells. 

 This is in harmony with the results of both Heymons (1895) an d 

 Carriere and Burger (1897) and justifies the statement made on 

 a previous page in regard to the homology of these cells. 



The ventral diaphragm is formed from muscle fibres arising 

 near the ventral longitudinal muscles, which extend out toward 

 the mid-line to join those of the opposite side. 



The structure and mode of development of the mesoderm just 

 described obtains throughout the region of the trunk, extending 

 from the middle of the second maxillary segment to the thirteenth 

 trunk segment. As has already been said, it is less modified than 

 in the head or in the extreme posterior abdominal region. In the 

 anterior region of the head, at Stages VIII and IX, there is 

 found a mass of somewhat loosely arranged mesoderm cells 

 filling the head cavity, the more central of which are applied to 

 the surface of the stomodaeal invagination (see Fig. 52 Meso). 

 Caudad of this region the mesoderm cells divide into two lateral 

 groups, and these in turn give place in the antennal region to a 

 pair of thin-walled sacs, each composed of a single layer of flat 

 cells. These are the coelomic (mesodermal) sacs of the antennal 

 segment (Fig. 84, Ant Meso). Each of these coelomic sacs sends 

 off laterad a solid prolongation into the cavity of the antennal 

 rudiment, filling the latter like a plug. The antennal coleomic sacs 

 diminish in size caudad and each finally terminates in a flattened 

 strand of mesoderm which traverses the mandibular and first 

 maxillary segment and in the second maxillary segment becomes 



