liESPIKATOKY ORGANS IN ARANE.R. 
29 
the younger ones (fig. 13b) — so much so, indeed, that in the 
dorsal region the latter project for at least half their mass 
into the body cavity, while the oldest saccules are still 
entirely contained within the appendage (a condition still 
apparent at the time of hatching, fig. 17). 
The plane of each saccule is still an inclined one, slanting 
upwards anteriorly, owing to the presence of the genital duct 
iu the now ventral (originally medial) portion of the appen- 
dage. When, in later stag’es, the duct has migrated else- 
where, the saccules come to lie horizontally and parallel to the 
ventral side of the appendage (figs. 17 and 18). A slight 
twist in the plane of a saccule may always be noticed in the 
o-G-furrow stage, by which each becomes distinctly more 
horizontal in its anterior region (fig. 13b) than at the orifice 
(fig. 13a). 'I'his twist does not seem to be retained through- 
out all subsequent stages. 
From the 5-furrow stage until the period when the 
cuticula and the lacuna; first appear in the lung-books the 
latter present various characteristics, best studied in trans- 
verse sections, such as fig. loB. The ventral wall of each 
of the saccules (.v. 1, .v. 2, etc.), is distinctly thicker than the 
dorsal wall, its cells being more cylindrical and more 
nursierons, its nuclei more oblong and situated nearer the 
ventral (basal) ends of tlieir cells, which thus come to have 
more protoplasm at the free (dorsal) ends than do the corre- 
sponding (or ventral) ends of the cells of the dorsal wall. 
'I’he saccules are each provided with a considerable cavity, 
but between the closely appressed walls of two adjoining- 
saccules no lumen whatever is found. 
^Vith the appearance of the chitinous structures and the 
blood-lacunae at the end of the reversion a great change takes 
])lace in the appearance of the walls of the saccules, the 
older ones being, as usual, those first affected. There appear 
between the walls and cells of two adjoining saccules irregular 
spaces figs. 17 and 18), which are at first small, but 
rapidly eidarge and communicate with one another and with 
the blood-cavities on the medial and lateral sides of the lung- 
