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 antennae of the latter, and probably 

 with the antennae 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 papillae be- 

 hind the antennae, 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 cov- 

 ered by a lateral fold of the germ band (Figure 9, p^i. or.). Sections 

 through the mandibles transverse to the germ band (Plate 3, Figure 16) 

 show that they are low broad ectodermal evaginatious 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 {pU. or.) which have grown more rapidly 

 than they ; and in the following stage (Plate 1, Figure 5 ; Plate 3, Fig- 

 ure 20, vid.), 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 (ind.) are swollen at their ends, their lateral surfaces conforming 

 to the adjacent surfaces of the folds {pli. 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, »if/.) the mandibles, 

 now wholly covered by the lateral folds {jtli. or.), are much longer and 

 still conical ; they are shorter and much more slender than the imder- 

 lying first maxillae ; 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 maxillae (Plate 5, Figure 29). In this stage the mandibular 

 muscles are individually distinguishable (Figure 32), and the anterior 

 extremity of the mandible l)cars several minute lobes (Figure 32), each 

 consisting of a single hypodermal cell. In the next (8th) stage the free 



