lee BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
tion of the hypodermis in Figure 1 — which is identical with the heavy 
black portion of Figure 2 — is destined to become the wing, and it is 
evident from an inspection of either of these figures that this organ is 
really a folded portion of the hypodermis itself, enclosing a thin layer of 
mesodermal tissue (mbr.m.). The conditions are, however, somewhat 
complicated. The wing-pad proper is a pocket-like owtfolding of the 
hypodermis, which, for want of room, is more or less folded upon itself. 
This pocket, instead of lying exposed between the hypodermal covering 
of the larva and its cuticula, is protected by being sunk into a deep sac- 
like trfolding of the hypodermis, the walls of which are very much thin- 
ner than those of the wing-pad, and indeed thinner than the rest of the 
hypodermis. The walls of the infolded sac follow quite closely the fold- 
ings of the wing-pad itself. It is evident that, in penetrating from with- 
out inward, one would traverse in the region of the wing-pad no less than 
five layers of the hypodermis: first, the outer and inner layers of the 
operculum-like fold of the hypodermis which covers in the wing ; then, 
in succession, the thick outer and inner layers of the wing-pad; and, 
finally, the thin inner layer of the infolded sac. 
The trachez (¢7., Plate 1, Figs. | and 2) penetrate between the two 
thickened layers of the wing-pad. The outermost of these two layers 
is destined to form the upper wall of the future wing, while the inner 
one becomes the lower wall of the wing. Figures 3 and 3a (Plate Ll) 
are representations of the histological condition of the cells which com- 
pose the wing-pads. Figure 3 is a portion of a cross section through the 
whole thickness (i. e. both walls) of the wing-pad, and Figure 3a is a 
small portion of a longitudinal section of the upper wall only. The cells 
are much more crowded in the longitudinal direction than they are in 
the direction across the wing. It is evident that these young wing-tissue 
elements are really spindle-shaped hypodermal cells; the nucleus being 
found in a swollen portion situated somewhere near the middle of their 
length. The inner ends of these spindle-shaped cells are often seen 
to be fused to a double membrane (mbr.m.), occupying the space be- 
tween the two walls of the wing-pad. In very old larvie, however, 
this membrane is usually absent, and the inner portion of the cells which 
constitute the wing tissue end free, as is seen in Figure 3a. The mem- 
brane, when present, forms a sort of sac, which encloses the trachez of 
the wing, and is continuous with the basement membrane which under- 
lies the general hypodermis of the larva. 
Pancritius (784) describes the development and histological condition of 
the wing in the larvee of several species of Lepidoptera, and his account 
