FOLSOM: MOUTH-PARTS OF ANURIDA MARITIMA, 135 
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pus is formed and resorbed, — an indication of degeneracy. The entire 
gular region of Apterygota is labial in origin; but fewer sclerites are 
differentiated than in Pterygote insects. The labium of insects is homo- 
dynamous with the “first maxille ” and homologous in detail with the 
first mavxillipeds of decapod Crustacea. The labium of Campodea is 
equivalent to the “‘second maxille ” of Symphyla, and is represented in 
the Diplopod gnathochilarium. 
Skull. 
The principal mouth-parts of Collembola, unlike those of all other 
insects, except certain Thysanura, are internal; the way in which they 
become so will now be described. 
The beginning of the process is seen at Stage 3 (Plate 1, Figure 3), 
when the ventral surface of the germ band is almost flat. In lateral 
aspect (Plate 2, Figures 9, 10) the edge of the germ band is produced 
downward as a small crescentic lobe (pli. or.) outside the fundaments 
of the mouth-parts. This lobe usually originates on the mandibular 
segment, as represented in Figure 9, but may arise more anteriorly, as 
in Figure 10. These figures represent, respectively, the left and right 
sides of the same individual. Rarely, the lobe begins behind the man- 
dible. A transection of the germ band near the middle of the lobe 
(Plate 3, Figure 16) proves the lobe (plz. or.) to be an evagination of 
the ectoderm enclosing mesoderm. In ventral aspect at this stage 
(Figure 11) the mouth-fold is clearly distinguishable at its widest part, 
or place of origin, but gradually disappears anteriorly and posteriorly on 
account of its confluence with the rest of the germ band. 
At Stage 4 (Plate 1, Figure 4) and a little later, while involution of 
the germ band is occurring, the mouth-fold is considerably larger (Fig- 
ures 12, 19, plz. or.) and forms a crescentic flap, now extending from the 
second maxilla almost to the labrum. In the next stage (Plate 1, Fig- 
ure 5) the fold is conspicuous ; in lateral aspect (Plate 3, Figure 20) its 
ventral margin is well rounded and conforms posteriorly to the con- 
tiguous anterior surface of the front leg ; the mandibular and maxillary 
fundaments still project slightly below the fold. In ventral aspect (Plate 
3, Figure 21) of the same individual, the fold is seen to be of nearly 
uniform width except anteriorly and posteriorly, where it is expanded 
against the labrum and second maxille respectively. Transections of 
the germ band (Plate 4, Figure 23), when compared with similar sec- 
tions at Stage 3 (Plate 3, Figure 16), show the folds to have exceeded 
