142 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



reticulum of the yolk. This plasmic sac has the form of an ellipsoid of 

 revolution or in longitudinal section that of an ellipse (Fig. 56, net). 

 The organisms gathered from various parts are caught in the meshes of 

 the plasmic net-like structure that is everywhere connected with the 

 plasmic reticulum of the yolk. After a time symbionts are no longer to 

 be found except on the net near which yolk cells also are to be seen. 

 From the apex of the stomodaeum ectodermal cells wander down the 

 apical plasma strand toward the yolk to form a cap-like structure over 

 the anterior end of the net-like bacteria-bearing sac (Fig. 56) . The net 



stom 



stom 



Fig. 56. — Hylobius abietis. Stom- 

 odaeal (ectodermal) cells (c) migrat- 

 ing over the anterior end of the 

 plasmic net (net), {stom} Stomo- 

 daeum. (From Scheinert.) 



Fig. 57. — Hylobius abietis. My- 

 cetom (myct) covering the limiting 

 membrane {lim. m) of the stomodaeum 

 (stom). (ect) Ectoderm, {ent) Ento- 

 derm. (From Scheinert.) 



gradually contracts, causing a diminution in the size of the meshes and a 

 concentration of the symbionts, drawing them toward the migrant 

 stomodaeal cells (c) which now constitute the mycetocytes. With fur- 

 ther contraction of the net the symbionts are forced into the mycetocytes 

 which thereupon form a compact cellular body (mycetom) attached to 

 the tip of the stomodaeum (Fig. 57, myct). The centrifugal gliding 

 migration of the mycetocytes on the closing stomodaeal membrane 

 (lim.m) and their pushing peripherally between the ectodermal and 

 entodermal cells finally form an annular mycetom outside the ali- 

 mentary canal at the junction of stomodaeum and mid-gut (Fig. 58, 

 myct), thus completing the larval mycetom. After this the union 

 between stomodaeum and mid-gut is restored. 



