600 ROBERT MATHESON 



large, cylindrical tube with a somewhat flattened epithelium iii 

 its anterior portion and distended with fluid from the mid-gui. 

 In longitudinal section the valve is never seen to form an open 

 cylindrical tube, but the walls, which in the larva project into the 

 mid-gut, now form a wide ring projecting at right angles to the 

 long axis of the alimentary canal and almost closing the lumen. 

 The epithelium of the posterior 'portion consists of rather tall, 

 narrow, cylindrical cells with well defined walls. The nuclei 

 are large and chromatic, are situated near the bases of the cells, 

 and do not show any indication of degeneration. The cytoplasm 

 is vacuolate and granular, the vacuoles being located near the 

 tips of the cells (fig. 21, v). At this stage no epithelial cells were 

 found undergoing division. A well defined basement membrane 

 is present and a pupal intima is formed though it stains but slightly 

 in eosin and not at all with iron-haematoxylin. 



The tall, glandular-appearing cells, situated in the larva at the 

 juncture of the fore- and mid-gut epithelium, have become 

 greatly reduced in size and show no signs of secretory activity. 

 They have become narrow, cylindrical cells like the rest of the 

 fore-gut epithelium. 



Throughout the entire fore-gut the epithelium is thrown into 

 longitudinal folds, but the folds are much reduced as compared 

 with those of the larval oesophagus. The lumen is closed a short 

 distance in front of the oesophageal valve, but appears again 

 slightly cephalad and continues as a small tube almost closed by 

 the longitudinal folds. The circular muscles are in a state of 

 active contraction. 



Changes from the time of pupation to the shedding of the pupal intima 



The next stage which requires special attention is that from 

 the time of pupation to the shedding of the pupal intima. The 

 time this occupies is dependent largely upon the individuals 

 examined, but requires tw^o to three days. Some time after pupa- 

 tion in the region formerly occupied by the larval gizzard, the 

 epithelium, which consists of greatly crowded cells, shows many 

 nuclei apparently undergoing chromotolysis (figs. 13 and 14, x). 



