Xn] EMBRYOLOGY 241 



6. The tracheal system arises from paired segmental ecto- 

 dermal invaginations. The original apertures persist, and become 

 the spiracles. The details of the embryonic growth of this system, 

 and of the first formation of the branchial basket of Anisoptera, 

 are unknown. In the embryo, the tracheal system is not filled 

 with air, but with a fluid whose origin is doubtful. The endo- 

 trachea and spiral thread are secreted in the embryonic period. 



7. The stomodaeum. We have already followed the develop- 

 ment of this invagination. It gives rise to the whole of the 

 epithelium and intima of the fore-gut, i.e. the buccal cavity and 

 pharynx, with the epipharynx and hypopharynx, the salivary 

 glands, ducts and reservoirs, the oesophagus arid crop, and also 

 the gizzard and its armature. Also, we have seen that a small 

 anterior portion of the mid- gut, probably corresponding with the 

 annular zone (p. 109), is derived from it. 



8. The proctodaeum. This has also been dealt with. It gives 

 rise to the epithelium and intima of the anus, the rectum and its 

 pads, the branchial basket of Anisoptera, the small intestine, 

 together with the Malpighian tubules, as blind outgrowths from 

 its extreme anterior end. A small posterior portion of the mid- 

 gut, probably corresponding with the sphincter region (p. 113), 

 is also derived from it. 



B. Parts of Mesodermal Origin. 



1. The heart is formed from two strings of cells called 

 cardioblasts, derived from the latero-dorsal walls of the mesodermal 

 somites. These strings become pushed up inwards dorsally, 

 until they meet and fuse in the middle line. They enclose between 

 them a pair of blood lacunae, which later on fuse to form the 

 lumen of the heart. The formation of the ostia, osteoles, alary 

 muscles and other details has not been studied in the Odonata. 



2. The coelome is the reduced remnant of the cavities of the 

 somites. It forms only the pericardium and the lumina of the 

 gonads. 



3. The blood-corpuscles are either somatic mesoderm cells 

 which have wandered into the haemocoele, or they may have 

 arisen from the yolk-cells. 



T. D.-F. 16 



