536 CAROLINE BURLING THOMPSON 



were drawn with the camera lucida from sections through the 

 central part of each mushroom body, and the fiber tracts recon- 

 structed from other sections. Figures 27 and 29 show the cells in 

 detail. 



Each mushroom body (figs. 12, 13) consists of two parts, an 

 inner lobe, and an outer lobe. The inner lobe projects slightly 

 farther forward and is a little wider from side to side than the 

 outer lobe, but in an antero-posterior direction, the two lobes 

 have the same measurement. Each lobe consists of an outer 

 layer of nerve cells and an inner fibrous portion, deeply indented 

 at its distal end," forming the cup or calyx, and continuing down- 

 ward as the stalk. The inner and outer stalks of each lobe unite 

 to form one common, main stalk (fig. 12, st.), which penetrates 

 deep into the fibrous core of each protocerebral lobe. The fibers 

 coming from the two lobes remain distinct throughout the greater 

 part of the stalk. 



a. The roots of the mushroom bodies 



The anterior roots of the mushroom bodies arise from the 

 main stalks just below the junction of the stalks from the inner 

 and outer lobes, in the region that is known as the 'crossing' or 

 'decussation' of the fibers (fig. 26). 



Careful examination of the sections in this region proves that 

 there is no crossing or decussation in the sense of the passing of 

 fibers from one side of the stalk to the other, but that there is a 

 gradual bending out of three or four very small bundles of fibers 

 from the longitudinal plane, that of the main stalk, into the trans- 

 verse plane. These four fiber bundles may be traced forward 

 and constitute the anterior roots of the mushroom bodies. Fig- 

 ure 24 shows the anterior root with four bundles of fibers, two 

 derived from the outer, two from the inner lobe. Some fibers 

 are seen passing between the root and the protocerebral tissue. 

 In figure 25, some distance anterior to the last figure, the two 

 bundles of each side have fused into one. An interchange of 

 fibers between the root and the protocerebral core is still taking 

 place and continues throughout the remaining length of the root. 

 It is impossible to state whether the direction of the fibers' is in- 



