TRICHOPTERA AND LEPIDOPTERA 347 



do not completely cover the dorsal side, except toward the caudal end. 

 The blind end of the proctodaeum, as noted above, is now entirely 

 free from mesoderm, though a few free cells remain near it. At 54 hours 

 the proctodaeum is about twice as deep as it is wide and shows the first 

 indication of the Malpighian tubes. Embryos at 59 hours show no indi- 

 cations of the formation of a mid-gut epithelium, though soon there- 

 after rudiments appear. 



The anterior rudiments are first visible at the posterolateral angles of 

 the stomodaeal membrane; the posterior mesenteron rudiments, at the 

 anterolateral angles of the proctodaeum near the origin of the Malpighian 

 tubes. By the sixty-fourth hour these rudiments have elongated ribbon- 

 like, so that the tips of the anterior pair growing backward have nearly 

 reached the tips of the posterior pair growing forward. Cells in mitotic 

 division are to be seen near the tips of these rudiments as well as more 

 remote from the tip. These ribbon-like strands of the mid-gut epithe- 

 lium are at first but a few cells in mdth (Fig. 307, mge). In structure 

 their cells do not differ from those of the stomodaeal membrane, nor is 

 there any indication of an interruption in the continuity of the tissues. 

 This condition is similar to that described by Schwartze (1899) for the 

 lepidopterous genus Lasiocampa and by Toyama (1902) for the silkworm 

 (Bombyx) as well as by a number of workers for species of other orders 

 and has given rise to the view that the epithelial layer of the mid-gut 

 arises from the ectoderm. 



The cephalic ends of the lateral parts of the inner layer give rise to 

 the mesoderm of the protocephalon; the lateral mesodermal parts, to 

 the muscles of the esophagus; and the median part, which originally lay 

 where the stomodaeum arises, is in part liberated as isolated cells and in 

 part pushed down by the stomodaeal invagination to form the sub- 

 esophageal body (Fig. SOSB,suboesb). This body has been interpreted 

 as endodermal by Hirschler et al., although Wiesmann (1926) regards it 

 as mesodermal. In Diacrisia this structure is purely embryonic, as it 

 is no longer to be identified after the end of the fourth day. The cells 

 that lie in the caudal pouch surrounding the proctodaeum give rise to 

 the muscles of the hind intestine. 



As has already been stated, the stomodaeum is feebly indicated at 

 45 hours, and 2 or 3 hours later the proctodaeum becomes evident. The 

 latter grows rather more rapidly, so that at 52 hours it is larger than 

 the stomodaeum (Fig. 303). The lumen of the stomodaeum becomes 

 slender, and the membrane forming the blind end quite thin. At 65 

 hours it is about 0.10 mm. in length, projecting inward nearly at right 

 angles to the axis of the body (Fig. 3065). Dorsally and laterally it is 

 surrounded by the mesoderm from which the muscle layers are developed, 

 ventrally by the unpaired subesophageal body (suhoesb) . At this stage 



