GENERAL MORPHOLOGY. 353 
The arborescent trachez 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 aérial 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 Trachew proper.— Great difference of 
opinion exists, however, as to the origin of the great longi- 
tudinal trunks and arborescent trachee. 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, A fis mellifica, he 
says: ‘In the next stage (i.c., after the formation of the 
stomodeum and the rudimentary maxilla 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 trachez.’ 
