DEVELOPMENT OF THE ALIMENTARY CANAL. 429 



In arriving at this conclusion, I have been guided by the 

 fact that the rectal valve is similar to the proventriculus in the 

 reduplication of the intestinal wall. It is well known that the 

 latter is formed as an invagination, and I have frequently 

 observed the distinct septum which separates the metenteron 

 from the proctodeum (PI. XXV., Fig. 6, s) both in dissections 

 and in sections. 



The Malpighian vessels and ducts are at first outgrowths 

 from the cells derived from Ganin's ring (PI. XXV., Fig. 2, m, mt), 

 and the proximal intestine only appears at a later date as a 

 thin straight transparent tube lined with columnar epithelium. 

 The muscular coat of the intestine is first seen as a sphincter 

 muscle close to the orifices of the Malpighian tubules, and then 

 the canal connecting the proximal and distal intestines appears 

 to be closed (PI. XXV., Figs. ^ and J). I have not been suc- 

 cessful in obtaining a section in the proper plane to show the 

 relation which subsists at this period between the lumen of 

 the distal and proximal intestines ; but if the intestine of a 

 pupa is dissected out and examined, a slight pressure on the 

 cover glass shows that the closure of the canal is due to 

 the above-mentioned sphincter, and that there is no organic 

 septum between the orifices of the Malpighian tubes and the 

 proximal intestine. 



At a later period, or from about the seventh to the eighth day 

 of the pupa, the distal intestine is filled and distended with a 

 clear coagulable fluid precisely similar to that contained in the 

 mesenteron; and judging from the appearance of the Mal- 

 pighian tubes, they have not as yet commenced to pour any 

 fluid into the intestine. They are quite twice the diameter 

 they have subsequently, are perfectly cylindrical, and of a pale- 

 yellow colour ; their cells contain both pigment granules and 

 minute spherules of oil in great abundance. The latter are 

 rendered intensely black by the action of osmium peroxide 

 (PI. XXV., Fig. <?). 



The rectal papillae have by this time attained very nearly 

 their final stage of development, and the rectal pouch is filled 

 with a milky fluid. This fluid is an emulsion of fine granules of 



