GENERAL .MORPHOLOGY. 353 



The arborescent tracheae are branching tubes which either 

 arise directly from the spiracular sacs or from the longitudinal 

 trunks or their commissures ; their finest branches are the 

 tracheal capillaries. In aerial insects both the arborescent 

 branches and the great longitudinal trunks and their commis- 

 sures are often replaced by chains of thin walled air vesicles or 

 sacs. 



b. Development of the Tracheal System. 



Development of the Spiracular Sacs and Spiracles. Observers 

 are all agreed as to the manner in which the spiracular sacs are 

 developed. They first appear as involutions or depressions of 

 the epiblast. The neck of the sac forms the external spiracular 

 opening, and the external valves are developed from its edges ; 

 a still further invagination of the epiblast, which occurs subse- 

 quently, forms the atrium, outside the external valve, when an 

 atrium is present. 



Development of the Tracheae proper. Great difference of 

 opinion exists, however, as to the origin of the great longi- 

 tudinal trunks and arborescent tracheae. Some hold that these 

 are, like the spiracular sacs, formed by invagination of the 

 epiblast, a view which was originated by Biitschli [126] ; and 

 others that they are developed from strings of mesoblast 

 (parablastic) cells. 



Kowalevski [97] says that in the embryo of Hydrophilus 

 tracheal pouches are formed by invagination, and subsequently 

 extend both forwards and backwards, and by their union form the 

 longitudinal tracheal trunks. He further insists : ' The pouches 

 not only form the stigmatic openings, but all the great tracheal 

 stems ' (p. 40) ; and of the embryo of the bee, Apis inellifica, he 

 says : ' In the next stage (i.e., after the formation of the 

 stomodaeum and the rudimentary maxillae and mandibles) 

 invaginations of the epiblast take place on each side of the 

 neural plate, which are the rudimentary stigmatic openings ; 

 these invaginations unite with each other and form the great 

 longitudinal tracheae.' 



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