200 THE EMBRYOLOGY OF THE HONEY BEE 



muscles (VLMcl). The somatic layer of the mesodermal tubes 

 now appears wedge-shaped in section, its broader end directed 

 mesiad and ventrad, and is moreover split longitudinally into an 

 inner thin single layer of cells, the rudiment of the dorsal dia- 

 phragm (DDph), and an outer portion composed of several 

 layers of cells, which are to form dorsal longitudinal muscles 

 (DLMcl). The yolk has continued to contract, drawing away 

 from the mesoderm over its entire extent so that the epineural 

 sinus is extended by the addition of two lateral cavities, which in 

 turn are in connection with the coelomic cavity by means of the 

 break in its visceral wall. These spaces together represent the 

 rudiment of the definitive body cavity. 



The process of differentiation goes steadily forward during the 



DDph..... 



DLMcl 



TraT 



Fig. 80. Transverse section through the third trunk segment of an 

 embryo, Stage XI, x 580. 



