138 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
case. Consequently the mouth-folds cannot represent the Crustacean 
second antenne. My own views as to the homology of the mouth-folds, 
already implied by my use of the term ‘genx,’ will presently be 
supported. 
Hansen’s recognition of the similarity between Campodea, Japyx, and 
Collembola is sustained by embryology. In Campodea, Uzel (’98, p. 33) 
‘describes and figures a “ Chitinstrang . . ., welcher sich von der Vorder- 
randmitte der zweiten Maxille um die Aussenseite des ersten Maxille 
und der Mandibel herum zu den auf den Intercalarsegmente gelegenen 
Hockern zieht.” His Figures 38 and 79 show the Chitinstrang at a 
rather advanced stage of development, corresponding with the condition 
in my Figure 17; unfortunately he gives neither its origin nor its earlier 
development. The later development, as evidenced diagrammatically by 
his Figures 80-84, agrees with that of Anurida in the gradual approxi- 
mation of the lateral ridges, and especially in the completion of the 
buccal boundary by the same method’ of confluence. Uzel does not 
attempt to explain the homology of the Chitinstrang. 
In Lepisma and Machilis the mouth-parts are ectognathous,.as in 
Orthoptera. In Lepisma there is no trace of a lateral mouth-fold, but 
in Machilis I have found a distinct, flat, lateral lobe sheltering the base 
of each mandible, and the lobe is probably homologous with the Collem- 
bolan mouth-fold. 
In Pterygota the gen, often not demarcated as distinct sclerites, 
represent the lateral regions of the germ band — as they do in Campodea 
and Collembola. In these Apterygota the same areas have simply in- 
creased as folds, but the folds are none the less homologous with the 
pleural regions of other insects, and in Collembola are reasonably to be 
regarded as the pleural portions of the premandibular, mandibular, and 
both maxillary segments. In many Pterygote insects, especially in 
Orthoptera, the gene overlap the bases of the jaws; for example, in 
Caloptenus, in which the gena is produced as a small but distinct flat 
fold over the base of the mandible. 
Little is known about the development of the sides of the head in 
Myriopoda, but in Peripatus it is interesting to find distinct lateral 
mouth-folds (Sedgwick, ’88, Plate IT., Figure 36) quite analogous, to say 
the least, with those of Collembola. 
Concerning the completion of the skull, little remains to be said. At 
Stage 7 a constriction encircling the blastoderm separates the head from 
the thorax. The head is typically a hollow cylinder, or cone, and so is 
the body. The body cylinder consists of a definite number of successive 
