THOMPSON: ANATOMY OF MOSQUITO. 
187 
remains of the annular muscles. By the tenth hour (pi. 16, fig. 43) 
the parts are hardly recognizable; for the valve (y) is flattened 
and almost obliterated while the cells of the annulus ( ann ) have 
largely lost their characteristic histological structure and resemble 
cells of the adjacent esophageal epithelium. The eleventh-hour 
pupa has almost the imaginal relations (pi. 16, fig. 40). The regen¬ 
erative ring and larval valve have disappeared, the epithelium is 
everywhere of esophageal type and near the cardia the walls are 
folding inward to build the imaginal esophageal valve. The rapidly 
diminishing belt of debris which remains from the degenerated 
annular muscles lies cephalad of the new valve. The infolded 
region clearly is epithelium that at one time formed part of the 
regenerative annulus. 
At about the tenth or eleventh hour the esophageal diverticula 
push out just caudad of the histolyzed annular muscles. They prob¬ 
ably do not involve cells that have formed part of the regenerative 
annulus, but arise from esophageal epithelium which has not been 
altered (pi. 16, figs. 40, 43 ,f res). The out-pushings quickly enlarge 
to small pouches, the ventral one opening widely into the lumen of 
the gut, the dorso-lateral ones with narrow stalks. Growth is then 
slow until the thirtieth hour of pupal existence, after which the 
pouches rapidly attain the respective imaginal proportions. The 
epithelium remains like that of the esophagus, columnar, till the 
very end of pupal life, when the characteristic imaginal epithelium 
is developed, seemingly by a flattening of the cells, accompanied by 
a folding and wrinkling of the walls. I am not certain whether 
these pouches secrete a pupal as well as an imaginal intima. When 
the mosquito leaves the pupa case the sacs are empty, but air 
bubbles appear shortly after emergence. 
It is not possible to escape the feeling that a relation exists be¬ 
tween the formation in Culex of the annulus of regenerative cells 
close to the cardia and the development of the ‘‘anterior imaginal 
ring ” at the same point during the metamorphosis of other flies. 
Such a ring of regenerative cells has been described for Chirono- 
mus, Antliyomyia, Stratiomya, Tanypus, Gastrophilus, and Musca 
(Vaney, :02). Its fate has been determined for Gastrophilus and 
Musca. In the former the whole stomodaeum is destroyed and is 
replaced “entirely by proliferation of cells derived from the buccal 
discs and the interior imaginal ring” (Vaney, :02). With Musca 
