liESPIKATOKY ORGANS IN ARANE.®. 
21 
parallel to the others and lies partly inside and partly outside 
of the pulmonary sac. Its medial part lies proximally to the 
lateral end of the second furrow, and, iu some cases at least, 
is continuous with the lateral end of the curved furrow 
mentioned above [gr,, fig. 14), which limits the appendage 
posteriorly. The fourth fui-row (/. 4, fig. IG) and all sub- 
sequent ones lie wholly within the pulmonary sac and appear 
successively as oblique grooves in its anterior wall, all more 
or less parallel to those already formed and with the medial 
ends of each lying on the proximal side of the lateral part of 
the previously formed furrow. 
After the appearance of the first two furrows the appen- 
da ges rapidly move from a ventral to a lateral position owing 
to the reversion of the germinal band, and it is necessary to 
bear in mind that we must substitute the terms “dorsal” and 
“ ventral” for “lateral” and “medial” after the lateral posi- 
tion has been I'eached.^ 
Tigs. 1-3 will make this clear. Fig. 1 is the position at the 
end of the 2-furrow stage; fig. 2 that at the end of the 3- 
furrow stage, and fig. 3 re])resents the position from the end 
of the 4-furrow stage, and here the appendages remain till 
near the close of the embryonic period. 'I'he whole segment 
which bears the appendage participates in this wandering, 
and the position of the appendage relatively to the adjacent 
surface is, of course, not affected by the movement. 
It will be observed that the youngest furrow (fig. 3) is the 
most dorsal one, and, if produced, would lie on the proximal 
side of all the older ones. 
'J'he pulmonary sac increases hand in hand with the forma- 
tion of new furrows, almost filling out the dorsal part of the 
hollow of the appendage. At the 5-furrow stage its blind 
end grows as a tube with a considerable lumen iu an upward 
or dorsal direction, raising up the outer epithelium as it 
pushes its way underneath (see hgs. IG, IGd, IGe). 
' For tlie sake of imiforiuity and in order to facilitate comparison 
between them, the sections of the earlier and later stages of the appen- 
dages have been drawn in the same positions throughout. 
