(80) 



520 0. GORDON HEWITT. 



layer of cells is proportionately much thicker. The cortical 

 tissue is made up of cells of varying sizes, but which can be 

 grouped in two classes smaller cortical cells and larger 

 ganglionic cells. Most of the ganglionic cells appear to be 

 unipolar, but there are many of a bipolar and multipolar 

 nature present ; they stain readily and possess fairly large 

 nuclei. These ganglionic cells are arranged segmentally, and 

 occur near the origin of the nerves. In the posterior region 

 of the ganglion, where the nerves arise in close proximity, the 

 ganglion cells are very numerous, relatively few of the cortical 

 cells being found. A further demarcation of the component 

 ganglia is brought about by median and vertical strands of 

 the ganglionic sheath -tissue, which perforate the compound 

 ganglion and occur as vertical strands along its median line. 

 Tracheae also penetrate the ganglion with these strands of 

 capsular tissue. 



On the dorsal side of the anterior end of the ganglion is 

 situated a pair of spherical structures (c. Z.), which may be 

 termed the " cerebral lobes." They are united in the median 

 line dorsal to the foramen traversed by the oesophagus (oe.). 

 These cerebral lobes are chiefly of an imaginal character, and 

 contain the fundaments of the supra-oesophageal ganglia and 

 also of the optic ganglia of the future fly (fig. 27). Each is 

 surrounded by a thin membranous sheath (sh.) and is con- 

 nected with the major cephalic imaginal discs by the optic 

 stalk (o.s.). 



The nerves arising from the ganglion may be divided into 

 three groups, according to their origin. Eleven pairs of 

 nerves (fig. 24, 1-11) corresponding to the eleven pairs of 

 ganglia arise, two from the anterior end and nine from the 

 sides of the ganglion. Three pairs of nerves (a., 6. and c.) 

 arise laterally from the stalks of the pro-thoracic and meso- 

 thoracic imaginal discs. In the median dorsal line of the 

 posterior half of the ganglion a single pair (d. a. ') and two 

 median unpaired (d. a. " d. a. '") nerves have their origin ; 

 these are accessory nerves. 



The first pair of the two anterior pairs of nerves runs 



