THE AMERICAN XA TURALIST. [Vol. XXXVIIl. 



chitinous plate underlying the oesophagus and protecting the 

 sub-oesophageal ganglion. From the anterior margin of the 

 body of the tentorium there extends forward a pair of processes, 

 the anterior arms, which are attached at the cephalo-lateral 

 angles of the clypeus, near the bases of the mandibles. Corre- 

 sponding with these are the posterior arms, which are near the 

 margin of the occipital foramen, just above the articulation of 

 the maxillce. In addition there passes up from the body of the 

 tentorium to be inserted at the margins of the antennal sclerites, 

 a third pair of processes, the dorsal arms. The entire structure 

 arises from two pairs of ectodermal invaginations. 



As early as the thirteenth day there appears cephalo-mesad of 

 the base of the mandibles a prominent pair of invaginations 

 which give rise to the anterior arms (Fig. 9, rt-. A). These 

 invaginations lie close to the latero-anterior angles of the man- 

 dibular ganglion. Heymons refers to the anterior arms in For- 

 ficula as originating at the base of the antennje. In Blatta they 

 are hidden by the antennae, which must be removed in order to 

 see the invaginations distinctly ; but they clearly belong to the 

 mandibular segment. 



The invaginations for the posterior arms lie cephalo-laterad of 

 the bases of the 2nd maxillas (Fig. 10, p.t) They are much 

 less prominent than those of the anterior arms, and, more or 

 less hidden by the ist maxilla, they are difficult to detect. They 

 seem to be somewhat retarded in their development, for I have 

 been unable to find them in embryos which clearly showed the 

 imaginations m the mandibular segment. 



I he invaginations of the anterior arms extend upwards along 

 the sidos nt the mandihulai ganohon, and then, bending at an 



'•summons tuse. and thus e 1 ilc'^, the mam bnd^ o\ tb. 

 >"^^""^>ni On the othei h md the posteiioi iina-maiions pa- 

 a- l>hiul Mcsshi^hth upuaul ami then toiuaid, to fuse ultimate' 

 with tlie antci-ior arms 



^^hileihc antuio, and the po tenor arms thus onginat. 

 in\ i,^inati..n> ot the body-wall, the dorsal arms arise, not a> n- 

 ^a.^iiuiion^ hut as processes, trom the anterior arms. These 

 l""-^-^ ..uhnlU extend dorsad and thus amie into conncc- 



