56 
W. F. PURCELL. 
body-epitlielium Iviiig in between {inf., fig. 28) folds into 
the body, and the two spii’acles become united to a single one 
{sp., fig. 28). 
Meanwhile, important changes hare taken place in the post- 
appendicular sac of the tracheal appendage. Fig. 28 repre- 
sents a reconstruction seen from above of the ectoblast of the 
two tracheal sacs (together with four muscles and two ento- 
chondrites) at the end of the reversion. Fig. 41 is a sagittal, 
and fig. 48 a transverse section of the same stage. The right 
lialf of fig. 28 is equivalent to the ti'acheal in-folding [fr. s.) 
in fig. 27, dorsal in the latter figure corresponding, of 
course, to lateral in fig. 28. 
We observe in the first place (fig. 41) the great longitudinal 
elongation of segment 9, bringing the tracheal spiracle 
and sac nearer to the posterior end of the abdomen. As if to 
compensate for this backward migration the medial region of 
each tracheal in-folding, that is, the region corresponding to 
the area ar. 9, fig. 27, to wdiich the entochondrite is attached, 
becomes drawn out in the form of an elongated plate (enta- 
])ophysis, ec. t. 9, figs. 41 and 28), which is directed forwards 
and slantingly upwards and is much compressed dorso- 
ventrally. 
8'he entochondrite (f.9) is attached to the narrowed anterior 
end of the plate, and at the extreme posterior lateral corner 
of the latter a second entochondrite (/., fig. 28) is found 
attached to those cells which bound the lateral angles of the 
spiracle. Between these two entochondrites the anterior 
oblique muscle («. oh. m. 10) is stretched. 
'I’lie bulging lateral jjortion {tr. prol.) of each plate cor- 
responds to the dorsal proliferation of a previous stage 
[tr. 2 n’ol., fig. 27). In the line of the transverse section, 
fig. 43 (see fig. 28), the lateral edge of the plate {tr. pil.) is 
some little distance from the outer hypodermis ( hy.) but 
more posteriorly, near the entochondrite t., fig. 28, the 
edge of the plate comes nearly or actually into contact, with 
the outer hypodermis. 
The pair of plates are connected like the spiracle by the 
