286 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



sacs. The subesophageal body, according to Strindberg, arises without 

 question from the mesoderm of the tritocerebral segment of the head. 



Tl^e development of the central nervous system, the stomatogastric 

 system, the endoskeleton of the head, and the tracheal system offers 

 nothing remarkable. 



The stomodaeum appears before the proctodaeum. Both, however, 

 from the start are clothed with mesoderm which is derived from a part 

 of the anterior and posterior ends of the inner layer. Those terminal 

 masses, according to Strindberg (1915), without doubt also contain 

 another type of ceils, the flattened entodermal cells which will invest the 



mge 



Fig. 216. — Sialis. Cross section of second thoracic segment, (cbl) Cardioblasts. 

 {dc) Provisional dorsal closure by amnion, {do) Secondary dorsal organ. (/) Fat body. 

 {ggl) Thoracic ganglion, (mge) Mid-gut epithelium, (mus) Muscle. 



blind ends of the stomodaeal and proctodaeal invaginations (Fig. 215, 

 mge). From each of these entodermal masses a pair of ribbons will arise 

 which laterally cover the yolk. In section (Fig. 216, mge) these sheets 

 appear as two thin crescents, each covered outwardly by the splanchnic 

 mesoderm. These lateral sheets elongate, those from the stomodaeum 

 meeting those from the proctodaeum, then widening to enclose the yolk, 

 first dorsally, then vent rally. The membranes at the blind ends of the 

 stomodaeum and proctodaeum in Sialis break down to establish the 

 communication between fore- and mid-, and mid- and hind-guts, shortly 

 before hatching. The rupture of amnion and serosa takes place about 

 as in the caddis fly, the serosa and the amnion forming the secondary 

 dorsal organ (do) which later is absorbed in the yolk. 



The development of this species has recently been described by 

 DuBois (1938) with special emphasis on the behavior of the embryo 

 under experimental conditions. 



