18 
W. P. PURCELL. 
follows so quickly upon the last that it is at first very puzzling 
to make out the changes accurately, hut with the aid of 
numerous reconstructions in wax 1 have been able to ascer- 
tain the more important phases with certainty. 
Fig. 14 is a sketch made from such a reconstruction, and 
represents the typical appearance of the right appendage 
seen somewhat from behind. Its distal surface is flat and 
often, although not always, distinctly transverse. Measured 
at the base, however, the breadth of the appendage is about 
equal to the antero-posterior diameter, and remains in this 
relation throughout the later stages. Seen fi’om the distal 
surface the appendage appears distinctly four-sided, with its 
posterior side placed transversely to the embx’yonic band. 
Fig. 8 is another reconstruction made from a series of 
longitudinal sections cut parallel to the principal axis {pr. ax., 
fig. 1) of the appendage, and a number of sections from this 
series are given in figs. 8a-8h, the positions of the sections 
being indicated by the vertical lines in fig. 8. 
'Phe first point to be noticed is the subsidence of the epi- 
thelium (ejx., figs. 8a-8g) lying immediately behind the first 
abdominal appendage and forming the posterior lip of the 
post-appendicular groove^ (yr., in stage 1, fig. 7). The two 
lips of the latter thus become drawn completely apart along 
its whole length, so as almost to obliterate the groove as such 
(except at a single place to be mentioned presently) and lay 
free the whole posterior side of the appendage. In its median 
Imlf the former bottom of the groove is now indicated only 
by a shallow furrow (yr., figs. 8a-8d), which at the same time 
marks what in the previous stage (fig. 7) was the base of the 
posterior side of the appendage. This shallow fui'row behind 
which the subsidence was greatest is moi-e or less curved 
owing to a shifting backwards of the tissue in which it lies 
(yr., fig. 14), so that the posterior side of the appendage 
comes to slant in its xnedial part at base (.v/., figs. 8a-8d, 
' A corresponding subsidence also takes place anteriorly to the fii-st 
ji])pendage, causing the obliteration of the groove between the seventh 
and eiglith segments. 
