102 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
third head somite. The tritocerebrum of Hexapoda is equivalent to 
that of decapod Crustacea, and the intercalary appendages of the former 
are homologous with the second antennz of the latter, and probably 
with the antenne of Chilopoda and Diplopoda. 
Mandibles. 
The fundaments of the mandibles appear in Stage 1 (Plate 1, Figure 
1, md.; Plate 2, Figure 8) as a pair of sub-hemispherical papille be- 
hind the antennz, and considerably nearer than they to the median 
plane. At Stage 2 (Plate 1, Figure 2, md.) they are longer and bluntly 
conical ; but at Stage 3 (Plate 1, Figure 3; Plate 2, Figure 9, md.) in 
lateral aspect they appear shorter than before, because the base is coy- 
ered by a lateral fold of the germ band (Figure 9, pli.or.). Sections 
through the mandibles transverse to the germ band (Plate 3, Figure 16) 
show that they are low broad ectodermal evaginations containing meso- 
derm. In Stage 4 (Plate 1, Figure 4; Plate 3, Figure 19, md.) the 
mandibles, although they have become long and cylindrical, are largely 
covered by the lateral folds ( pl. or.) which have grown more rapidly 
than they ; and in the following stage (Plate 1, Figure 5; Plate 3, Fig- 
ure 20, md.), though still nearly perpendicular to the germ band, they 
are almost completely covered laterally by the folds. The mutual rela- 
tions of mandibles and folds are shown in transections of the germ 
band (Plate 4, Figure 23), in which it may also be seen that the man- 
dibles (md.) are swollen at their ends, their lateral surfaces conforming 
to the adjacent surfaces of the folds (plz. or.). The long axes of the 
mandibles converge at their bases toward the median plane, and it is 
noteworthy that the lateral surface of each mandible is distinctly longer 
than the mesal surface (Figure 23, md.)—a foreshadowing of the 
oblique orifice of the finished organ. 
At Stage 7 (Plate 2, Figure 7; Plate 4, Figure 24, md.) the mandibles, 
now wholly covered by the lateral folds (plz. or.), are much longer and 
still conical ; they are shorter and much more slender than the under- 
lying first maxillee ;-and instead of being perpendicular to the germ 
band, they have now swung forward through an angle of almost ninety 
degrees ; moreover, they converge in front toward the median plane, as 
do the first maxille (Plate 5, Figure 29). In this stage the mandibular 
muscles are individually distinguishable (Figure 32), and the anterior 
extremity of the mandible bears several minute lobes (Figure 32), each 
consisting of a single hypodermal cell. Inthe next (8th) stage the free 
