KESriKATOKY OKGAN<>; IN AUANlvE. 
57 
in-foldiug {inf,, fig. 28) of the epithelium between them, and, 
therefore, possess a common lumen, which, however, is con- 
fined to the basal region only and denoted in the figures by 
the area, tr. I, within the dotted lines. The greater part of 
each plate is, therefore, solid. Fig. 28 gives the correct out- 
line of the pair of plates, as they appear near the end of the 
embryonic period, and I have tested the accuracy of the 
reconstruction by comparisons with sections cut parallel to 
the tracheal plates, so as to contain the whole width of the 
])air of plates in one section. I could not detect a distinct 
chitinous lining within the lumen of the plate at this stage, 
for the cuticula which previously covered the body surface 
always appeared quite loose and outside of the cavity, as if 
the embryo were undergoing a moult such as Locy describes 
for Ageleua. 
If the tracheal and pulmonaiy appendages be now com- 
pared, the diffei'ence in the relative development of the two 
main organs connected with them becomes apparent. The 
entapophysis is small in the pulmonai-y segment but large in 
tlie tracheal segment, where it forms the greater part of the 
tracheal plate, while the large mass of cells composing the 
lung-book is represented by the comparatively small, lateral, 
bulging portion of the traclieal plate. 
The post-embryonic development of the tracheal plate. — After 
the hatching of the embryo very important changes take 
place in the shape of the tracheal plates. In the first place 
the medial tendinal portion of each becomes drawn out in a 
forward and upward direction to form an elongate, spathulate, 
hollow process, which is strongly flattened dorso-ventrally 
and much broader anteriorly than in the middle. Its shape 
may be seen in fig. 29, wliich gives an accurate representation 
of the pair of plates after the first post-embryonic moult 
(stage 8). 'I'he tracheal lumen {tr. 1.) now extends to near 
the anterior end, where it is also broader, but since the dorsal 
and ventral surfaces are practically in contact (fig. 29a) this 
portion can scarcely function as a respiratory organ at this 
stage. 
