THE ALIMENT A RY CANAE 57 



section through the proventriculus shows that it consists of 

 three layers, the outermost continuous with the wall of the 

 chyle stomach, and the innermost with that of the oesophagus. 

 By traction the cesophagus can be drawn out wath the inter- 

 mediate layer, demonstrating that the organ is formed by an 

 intussusception or intrusion of the cesophagus within the 

 mesenteron. 



Tracheal vessels and mesoderm cells pass into the space 

 between the inner and intermediate layers of the proventriculus; 

 and the cesophagus is surrounded by a tracheal ring, from which 

 these vessels arise, with twelve others, which course over the 

 exterior of the proventriculus and chyle stomach. 



The epithelium, in the interior of the proventriculus, is 

 divided into two regions by a distinct ring of embryonic cells 

 (PL I., Fig. 4, id), which is destined to develop in the nymph 

 into the proventriculus of the imago. I shall term it the pro- 

 ventricular ring. The cells of the external wall are cubical ; 

 those of the internal wall exhibit a peculiar feathered appear- 

 ance, and are not unlike the epithelium of the human stomach. 



Kowalevski* regards the proventricular ring as the rudiment 

 of the stomodseum of the nymph. This is an error, due to the 

 prevalent view that the whole of the proventriculus is 

 stomod^al. I shall hereafter show that it forms the proven- 

 tricular epithelium only. 



A large crutch-shaped ganglion (PL I., Fig. 3, g), the 

 provcntricidar ganglion, lies in the angle between the dorsal 

 surface of the oesophagus and the proventriculus, at the 

 posterior end of the stomogastric [median) nerve ; ganglionated 

 fibres pass from it into the wall of the proventriculus and 

 chyle stomach. 



The Chyle Stomach (PL I., Figs, i, 3 and 6), is 3 mm. long, and 

 broader in front than behind. It has four tubular caecal 

 glands at its anterior end. These glands measure about 2 mm. 

 in length, and resemble the chyle stomach in structure. They 

 do- not reappear in the imago. Its epithelial coat consists of 



* 'Beitrage zur Kenntniss der Nach-Embryologie der Musciden.' Zeitsch. 

 f. w. Zool., Bd. xlv., 1887. 



