364 



with a mass of mesodermal padding tissue, which evidently 

 acts as a sufficient protection for tho delicate nerve fibres. 



It seems scarcely necessary to point out again that the 

 large cell lying beneath the tactile bristle is not a nerve cell, 

 as it is usually believed to be, but that it is merely the ecto- 

 dermal cell (receptor cell), from which the bristle has been 

 formed. The cell is not an element alien to the bristle, but 

 rather is the bristle to be regarded as a special part of the 

 cell which acts as an intermediary between the cell and the 

 environment, much as do the taste -hairlets at the free ends of 

 the cells of mammalian taste-buds. 



The Male Go'pulatory Organs. — The visible cellular 

 changes which underlie the formation of the penis are very 

 simple; first the cells, in the early pupa, adopt the usual 

 columnar (growing) shape; in the pupa two days old they 

 become cubical, and chitinise a day later. 



Tlie cells of the anterior dilation of the cavity of the 

 penis, the vesicula seminalis, are quite different in shape; 

 they are long and narrow, and form a thick wall around the 

 vesicle. 



The An-tennae. — At the front of the head are formed a 

 pair of appendages, the antennae, which are quite different 

 in nature from those appendages above described. Below 

 them an outgrowth of nerve fibres is formed from the brain, 

 on each side; and the fibres growing into the developing 

 antennae terminate on modified ectodermal cells in these, 

 the modifications of the ectodermal cells into sense cells, and 

 indeed, of the whole ectoderm into an antenna being of such 

 a nature as to form what must be a very efficient sensory 

 structure. 



The antennae are present in the earliest larvae — indeed, 

 at this stage they are already more advanced than are the 

 legs or wings, each being now in the form of a small papilla, 

 composed of long narrow cells undergoing evagination from a 

 previously invaginated antennal disc (fig. 47). They are, 

 indeed, at a stage of development which the legs do not reach 

 till the end of larval life. 



During the larval period the cells of the rudimentary 

 disc continue to divide, so that, shortly before the larva 

 defaecates, a distinct antenna is visible on the surface of the 

 larva (fig. 3). Already at this stage the curiously jointed 

 condition of the mature structure is clearly indicated (fig. 36), 

 for the ectodermal cells have not divided regularly, as is the 

 case in the legs and wings, but at short intervals a few cells 

 have ceased to divide for a time, and remain long and 

 columnar, while between these the ectoderm has undergone a 

 marked proliferation to form a solid downgrowth of very 



