268 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
involution of the head is in progress. Three pairs of thickenings 
of nearly equal size appear in the ventral ectoderm of the forward 
end of this embryo. In the next stage shown (PI. 6. Fig. 35), in 
which the involution of the head is completed, these thickenings 
have begun to invaginate, and three additional pairs of thickenings 
have made their appearance in the dorsal ectoderm at the forward 
end of the embryo. Only the dorsal prothoracic thickening ap¬ 
pears in Figure 35 ; it lies immediately in front of the dotted line 
leading from the letters else. ce. The invagination of each ventral 
disc begins at its posterior border, as shown in the ventral meta- 
thoracic disc in Figure 35; then the anterior border sinks in, as 
shown in the mesothoracic disc in this figure ; finally the entire disc 
sinks beneath the surface, as is shown in the prothoracic disc in the 
same figure. As this figure clearly indicates, the invagination of 
the three discs is not simultaneous, but the more anterior the disc 
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the earlier the invagination. Complete invagination rapidly follows 
(PI. 7, Fig. 45) ; the disc at once separates itself from the ecto¬ 
derm, and the opening made in the ectoderm by the invagination 
closes (PI. 7, Fig. 46). It is while the discs are in this condition 
that the embryo leaves the egg-envelopes, and the discs remain in 
this condition during the entire larval life of the insect. 
The dorsal thoracic discs do not invaginate during the embryonic 
life of the animal. The meso- and metathoracic discs invaginate 
in the young larva, and become detached from the ectoderm, as 
do the ventral discs (PI. 7, Fig. 46). The prothoracic discs do not 
advance beyond the stage of development represented by figure 45, 
but remain thick-walled pockets of the ectoderm. They are also 
lined with a cuticula, continuous with that of the rest of the larval 
ectoderm, and the different moultings take place from the pockets 
as from the rest of the insect. In my preliminary paper (Pratt, 
’97) I have stated that the dorsal prothoracic discs have a larval 
origin, but later examination of the material has shown this to be 
an error. As is the case of the cephalic discs, mesoderm does not 
appear in the thoracic discs during the embryonic life of the insect, 
nor do nerves or tracheae enter them. 
3. The Discs of the External Genitalia. — These discs first 
appear in the embryo shortly before it leaves the egg-membranes. 
Two pairs of ectodermal thickenings then appear immediately 
in front of the anus, — a larger posterior and a smaller anterior 
pair. The former lie just in front of, and partly embracing, 
