I 
26(3 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
The cephalic imaginal discs now grow rapidly, and soon attain 
the form which characterizes them in the young larva. The median, 
ventral disc has begun to invaginate (PL 6, Fig. 34, else. ce. m.), and 
a cross-section through this region (PI. 6, Figs. 36 and 37, else. ce. m.) 
shows that the invagination is paired. (Compare these cross-sections 
with Figure 34, where the positions of the cross-sections shown in 
Figures 36-44 are indicated by the corresponding numbers, 36-44.) 
The later history of this disc (see Pratt, ’93) shows that these paired 
outgrowths of the ventral invagination develop during the larval 
period into deep pockets, that a long projection springs from the 
bottom of each of them, and that subsequently the pockets fuse in 
the median plane and the projections develop into the proboscis of 
the imago. 
The muscular sucking tongue is now an internal organ (as may 
be seen in Figures 34, 37 and 38), and projects free into the 
pharynx. Figure 39 shows the base of the sucking tongue where 
it>is continuous on each side with the pharyngeal wall. The open 
space dorsal to it is the forward end of the median portion of the 
lumen of the dorsal discs (of. m.), where it communicates with the 
pharynx. In the stage of development represented by Figure 30 
(PI. 5) , this opening (of. m.) leads to the outside of the animal’s body. 
The paired nervous masses below the oesophagus are portions of 
the circum-oesophageal or cerebral commissures (Fig. 39, corns, cb). 
The section shown in Figure 40 (PI. 7) also passes through the 
base of the sucking tongue. The median portion of the fused dor¬ 
sal discs (else, ce.) is much broader here, extending quite across the 
embryonic body, while between it and the oesophagus (oe.) are 
the muscle-mass of the tongue (Ing.) and the cerebral nerve- 
mass (e/n. cb.). The latter is in close contact with the ventral sur¬ 
face of the disc, and with the tongue muscles quite encloses the 
oesophagus. Figure 41 represents a section made immediately back 
of the sucking tongue. The median portion of the fused dorsal 
discs (else, ce.) is still shown, and between it and the oesophagus are 
the median nerve (n. rn.) —which joins the sucking tongue with the 
ganglia allata — and the two cerebral nerve-masses (gn.cb.). Fig¬ 
ure 42 shows a section considerably farther back (compare Fig. 34), 
passing through the paired portions of the dorsal discs (else, ce .), 
and also exhibiting the structures seen in the previous section. The 
next figure (Fig. 43) shows the paired portions of the dorsal discs 
(else, ce.) distinct from each other, though still in contact with the 
