365 



minute cells; from the latter the tissue which produces the 

 bulging of the joints is formed — a kind of padding tissue — 

 whilst the former, the long columnar cells, give rise eventually 

 to the joints and constrictions between the segments of the 

 antenna. 



Already at this early stage a distinct outgrowth of nerve 

 fibres from the brain is seen, though it does not as yet extend 

 right into the developing antenna; a lars^e tracheal vessel 

 can also be distinctly seen, at this stage, within the lumen 

 of the antennal projection (fig. 36). Leucocytes have also 

 begun to enter. 



The mesoderm still adhering to the overlying ectodermal 

 cells, follows them as they grow outward to form the antenna. 

 The cells do not appear, at this stage, to have undergone 

 any marked proliferation, and consequently appear as a deli- 

 cate network lining the lumen of this structure. But that the 

 mesoderm does eventually proliferate seems quite clear. This 

 will be referred to later. 



The post-defaecation period of the larva is marked by a 

 continuation of this process; the antenna growing rapidly 

 eventually attains the siize that we see in the mature insect. 

 The differentiation is, as yet, however, very incomplete. The 

 cells of the "padding tissue" have proliferated so rapidly that 

 the mass develops a temporary invagination cavity (fig. 36). 

 The columnar "partition cells" have divided, and now form 

 a narrow ring of short cells between the masses. The ecto- 

 dermal cells then partake in the process of cuticle secretion 

 which is going on everywhere in the integument at this stasfe, 

 a process which, when it is complete, is immediately followed 

 by a shedding of the larval cuticle. 



The antenna now contains two tracheal vessels (fig. 65) ; 

 large numbers of leucocytes are present, and the nerve out- 

 growth from the brain has extended practically to the tip 

 of the structure. 



A rapid differentiation now takes place. The cells com- 

 prising the integument of the antenna adopt a more regular 

 arrangement; the cells which give rise to the joints between 

 the segments and to the proximal and distal walls of the 

 segments proliferate somewhat, and, perhaps, on account of 

 the presence of a hard cuticle on their outer surface, grow 

 inwards, dividing the whole antenna into the segments so 

 characteristic of that stage. This condition, the commence- 

 ment of which is seen in the four-hour pupa, is complete in 

 the pupa thirty-six hours of age (fig. 15). 



Meanwhile the formation of bristles has been taking 

 place. Already in the four-hour pupa a number of integu- 

 mentary cells at the tip of the antenna have elongated and 



