544 CAROLINE BURLING THOMPSON 



higher level, probably from the cells of Group III. It is impos- 

 sible to state whether the fibers all run in the same or some in 

 the opposite direction from side to side. 



The paired tracts g are present in both lobes, originating in the 

 cells of both Groups II and forming the main bulk of each stalk 

 in the two parallel bundles gl. and gr. Tract h arises in both 

 lobes from the cells of Group I. Its fibers form a fan-shaped 

 mass, rapidly converging into a slender bundle which occupies 

 the center of the stalk, bounded on the outside by the paired 

 bundles of gl. and gr. Tract m originates in Group III, r. of the 

 outer lobe. Its fibers run downward on the outer surface of the 

 stalk. 



Camponotus worker: figure 33. The first tract to be found in 

 the worker is b, homologous with that of the queen, but in this 

 caste its origin can be determined. Tract b originates in the 

 large nerve cells of the cortex of the mandibular lobe. Bundles 

 of fibers can be seen leaving these cells, entering the fibrous core 

 of the mandibular lobe, passing upward into the outer part of 

 the protocerebral lobe and thence into the bundle which like 

 that of the queen cui'ves in front of the outer and inner stalks 

 of the mushroom body, in contact with its fibers, and finally 

 passes toward the median line in a curve. Then downward, on 

 its way passing through the anterior protocerebral commissure, 

 and going deep into the protocerebral tissue near the central 

 body. Tracts c and d are absent in this worker, tract e is sim- 

 ilar to that of the queen. Tract fl, which probably represents 

 both / and I of the queen, is a single commissure uniting the 

 inner and outer lobes. The dorsal part, apparently arising from 

 the two Groups IV, is slightly denser than the loosely ar- 

 ranged ventral part, which arises from a higher level. At both 

 ends the fibers may be seen to arise from the cells of Groups III 

 and IV. It is therefore evident that some fibers must run in 

 each direction in this tract. Tract h and the paired tracts gl. 

 and gr. are similar to those of the queen. Tract n, present in 

 two sections, is a very slender but distinct fiber bundle. It orig- 

 inates in Group III, r of the outer lobe, and leaving the stalk, 

 terminates in the outer part of the protocerebral tissue. N may 



