24 



PSYCHE. 



[Febri 



these organs. They giadually migrate 

 towards tlie two poles of the egg, and 

 finally attain positions slightly ventral 



Fig. 5. Embryo Melophagus,side view 

 I', ventral side; Sy stomatodeum ; P, proctodeum: VP^ 

 ventral plate ; 7", tracheal invaginations ; H^ head-fold. 



to the anterior and posterior poles 

 respectively. Long before the stom- 

 atodeum has attained its definitive 

 ]iosition, however, there appear before 

 and behind it crescentic, ectodermal 

 thickenings which partly encircle it as 

 shown in Fig. 6. These are the begin- 



Fig. 6. Anterior end of embryo, do 

 fold; J', stom.itodevmi ; I'D, ventral dis 

 discs. 



•w ; //, head. 

 riD. dorsal 



nings of the cephalic imaginal discs, 

 which, it will be seen, appear very 

 early in the ontogeny of tlie insect. 



while, in fact, the intestinal tract, the 

 tracheal, and nervous systems are in 

 their first anlagen. The crescentic thick- 

 enings, now, are three in number, a 

 pair just behind (D D, Fig. 6) and a 

 single median one just before(VD, Fig. 

 6) the stoinatodeum. The latter thick- 

 ening is destined to form the ventral 

 cephalic disc ; the fate of the pair, on 

 the other hand, is to form the dorsal 

 discs, those homologous with the ceph- 

 alic discs of Musca. At the posterior side 

 of each of the latter thickenings, now, an 

 invagination begins to appear which 

 finally becomes a deep pocket. Each 

 pocket early shows an intimate connec- 

 tion with a supra-oesophageal ganglion, 

 the latter abutting the posterior side 

 of the pocket, but having no structural 

 union with it. During the formation 

 of these pockets, however, the stoma- 

 todeum, together with the doisal and 

 ventral discs, has been continuing its 

 migration towards the anterior end of 

 the embryo, and the formation of the 

 intestine and closure of the back of 

 the embryo have been going rapitlh' 

 forward. Finally, when the mouth has 

 arrived at its definitive position at the 

 anterior pole of the embryo, the dorsal 

 pockets have come to occupy a position 

 on the dorsal side of the embiyo just 

 above it, and the back is entirely closed. 

 The openings of the pockets have by 

 this time moved to the mid-doisal line 

 and merged into a single median open- 

 ing. In Fig. 7, DD represents this 

 common opening. The proximal or 

 ujjper portions of the pockets have also 

 fused, but their distal ends are still free 



