439 



and backwards of the head, as already described, and the- 

 result is a total change in the course taken by the oesophagus. 

 It now passes not directly backwards, but first upwards and 

 forwards actually, and only then begins gradually to bend 

 backwards. Meanwhile the cells of the circumoesophageal 

 imaginal ring have continued to proliferate, and in the fresh 

 pupa form a great cone of cells, attached behind to the 

 anterior end of the foregut, and ending, in front, just behind 

 the brain (fig. 154). The structure is composed of two layers 

 — an_inner of long columnar cells, all tightly compressed and 

 arranged "radially around a very narrow central lumen. Out- 

 side this is a second layer one or more cells in thickness} 

 the individual cells are much smaller here (fig. 153). From 

 the great inner layer the succeeding portion of the intestine 

 as far back as the gizzard is soon to develop. 



The smaller outer layer has a much humbler future ; when 

 the cells of the great inner layer have migrated backwards 

 (a process which commences several hours after pupation), 

 the outer layer cells extend round to the lower side of the 

 oesophagus and form a rather thick column there in front of 

 the neck, but not extending as far downwards as the brain. 



It is in connection with this structure that the developing 

 myoblasts of the rear of the oesophagus now come; these 

 cells, having united end to end during late larval life, now 

 form several rows of cells, inserted behind all upon the sub- 

 oesophageal cell column, and in front at various points on 

 the rear of the oesophagus. Adjacent cell walls now break 

 down, and each row forms a single multinucleated syncytium. 

 During the third day of pupal life these syncytia develop 

 spiral striations and form the post-oesophageal muscles of the 

 imago. Meanwhile, the sub-oesophageal cell column, upon 

 which the columns of myoblasts are all inserted, begins, in 

 the thirty-six hour pupa, to chitinise internally. Chitinisation 

 continues and the chitin rod fuses with two other very short 

 processes which have grown out from the rear of the head, 

 close to the neck. By this means, the musculature of the rear 

 of the oesophagus obtains a very firm support. 



Already in the fresh pupa, the myoblasts of the anterior 

 part of the oesophagus have disposed themselves longitudinally 

 or circularly in the position they are to occupy in the imago ; 

 several cells usually fuse to form small syncytia, and, under- 

 going the usual differentiation, form the striated muscles of 

 the adult oesophagus. 



It remains only to note that these changes are accom- 

 panied by a new secretion of chitinous cuticle within the 

 lumen of the oesophagus, the old having been drawn out 

 through the mouth at the pupal moult. The development of 



