MOUTH PARTS OF THE EMBRYO. 



Hvpopharynx. The hypopharynx is evident near the dorsal base of the 

 labium on the eighth da)'. It then consists of three single papillae : a median 

 papilla between the bases of the two maxillae ; and a lateral papilla at either side 

 near the bases of the mandibles. With the forward migration of the mouth- 

 parts, they are crowded together. At hatching, the hypopharynx (PI. I, fig. 13) 

 is a trilobed structure with sensory hairs. 



First Maxillae. The first maxillae arise as simple cone-shaped papillae 

 (PI. I, fig. 5, mx.l.) similar to those of the mandibles, but they elongate more 

 rapidly. A lobe soon appears on the outer lateral region (PI. I, fig. 14, ol). 

 This is the future palp. The apical region becomes rounded and flattened. Then 

 this area subdivides into two parts (PL I, fig. 15, oa. ia). The outer apical 

 region differentiates into the galea; the inner into the lacinia. As the galea-lacinial 

 portion elongates, the cleft between the two parts increases. On the eighth day 

 segmentation in the palp and cardo-stipital region is indicated by notches (PI. I. 

 fig. 16). Galea and lacinia gradually fuse. The cardo becomes distinct, but 

 neither the palpifer nor the suture between the stipes and galea-lacinia is differ- 

 entiated. Spines appear as cone-shaped papillae at the distal part of the galea- 

 lacinia on the tenth day. At hatching, hairs are evident on the distal palp segment 

 (PI. I, fig. 17, Mx. Pip), and the inner galea-lacinial region (PI. I, fig. 17, Ga Lc). 

 This part is chitinized heavily ; the palp very weakly. 



Second Maxillae or Labium. The second maxillae arise as cone-shaped 

 papillae on the fifth day (PI. 1, fig. 5, Mx2). They become bi- and tri-lobed 

 (PI. I, fig. 18) at the same time as the first maxillae. On the seventh day a 

 secondary, lower-median lobe appears (PI. 1, fig. 19. lm) and the two second 

 maxillae migrate toward the mid-ventral line. The secondary lobes fuse first 

 (PI. I, fig. 20, lm). At that time the galea and lacinia are distinct (PI. I, fig. 20, 

 Ga, Lc). There are also traces of segmentation in the palp (PI. I, fig. 20, Lb 

 l'lp), and indications of the submentum below the fused lower-median lobes. 

 The large central portion becomes the mentum of the nymphal labium (PL I, 

 fig. 21, M), the lacinia the glossa (gl) and the galea the paraglossa (Pgl). A 

 third distal segment of the palp is incompletely differentiated (PL I, fig. 21, Lb 

 Pip). Spines and hairs are elaborated at the same time as those of the first 

 maxilla. 



The early appearance of the inner lobe on the mandible, and the fact that 

 it is articulated, but has no musculature, suggests the possibility of a primitive 

 structure. It occupies the same relative position on the mandible as the lacinia 

 mobilis of certain Crustacea (Mysis, Arolana). Embryological evidence indicates 

 that the movable inner lobe of the May-fly mandible is a lacinia. 



A mandibular palp is present at no stage of embryonic development. 

 Ephemera vulgata used by Heymons for his embryological study is a burrowing 

 May-fly. The mandibular tusk, which he calls the "morphological equivalent of 

 a mandibular palp" is a secondary modification appearing at the time of differ- 

 entiation of the canines and molar region. It is not lateral in origin, but arises 

 from the outer apical region. 



7 



