362 



which have become specialized into secreting a certain struc- 

 ture may apparently (perhaps as a result of the act of secre- 

 tion) become modified so as to resemble neighouring cells, and 

 then co-operate' with these in the secretion of another (some- 

 times unlike) structure. The observation, if correct, would 

 be of considerable theoretical importance. I cannot, however, 

 state with certainty whether these earliest specialized cells 

 persist throughout larval life. 



Early in the resting period the mandibular palp already 

 mentioned above is distinctly seen. The disc grows rapidly 

 by mitotic division of the cells, and drags the mesoderm 

 after it. 



The remainder of the development of this, and of the 

 other appendages of the mouth, closely resembles that of the 

 legs, and need not be described further here. It is only 

 necessary to add that the imaginal discs of the first and second 

 maxillae are present in the earliest larvae, and do not differ, 

 except in position (being closely applied to the mouth) from 

 the leg discs. 



The Ovipositor. — The early stages in the formation of the 

 ovipositor are identified with those of the legs, i.e., the 

 imaginal rudiments, present in the early lar\^ae as ectodermal 

 thickenings, become invaginated into the abdominal cavity 

 (fig. 2). In the resting period they grow outwards, ■drag'ging 

 the mesoderm after them; rapid cell division, by mitosis, 

 results in the great extension of these appendages, along the 

 ventral body wall, as has already been described fully above. 

 The ectoderm of these appendages is only one cell-layer in 

 thickness, and the cells themselves are generally long and 

 narrow. The third appendages differ from the others, how- 

 ever, in being several cell-layers in thickness ; they are not 

 hollow, as are the others, and are fused with the body wall. 

 Into the hollow appendages, as usual, migrate leucocytes, 

 which disintegrate there during the pupal stage. The second 

 appendages in the larva seven hours after defaecation have 

 already become closely approximated, and the cells on their 

 adjacent halves have forsaken their long columnar shape and 

 are now cubical ; this portion of the appendage is in process of 

 invagination into the outer part. 



In the early pupa, the cells of the developing ovipositor 

 begin to lose their columnar shape, the characteristic of the 

 growing stage, and now beco'me cubical, and rather small in 

 volume, i.e., the ovipositor as a whole, having reached its 

 condition of maximum growth, now begins to differentiate. 

 The cells, themselves, become more closely packed together, 

 and the long appendages shrink, just as we saw, above, in 

 connection with the legs and wings. 



