284 CAROLINE* BURLING THOMPSON 



seen passing between the central body, and the mushroom body 

 stalks and protocerebral tissue, pi. 



In figure 2 the upper part of the central body is the same as 

 in the preceeding section, but the lower part is divided by the is- 

 suing and entering fibers into two lobes or masses of fibers. The 

 posterior surface of the mushroom body stalks is shown in this 

 section, and fibers coming from the inner division of the stalk are 

 seen in longitudinal section, curving up into the central body and 

 forming the ''central body roots" of the mushroom bodies, c.b.r. 

 Penetrating between these root fibers are other fibers that run 

 between the central body and the protocerebral tissue. 



In figure 3 the upper part of the central body, c.b., is disappear- 

 ing, and consists of disconnected masses of fibers in cross 

 section and bundles of longitudinally cut fibers that are evi- 

 dently making their exit to the protocerebral tissue. The two 

 lower lobes or fiber masses are still present on each side of the 

 median line. Lateral to each of these are two bundles of fibers 

 in cross section, p.r., derived from the mushroom body stalks, the 

 outermost bundles still coimected by longitudinally running 

 fibers with the inner part of the stalks. These bundles, which are 

 issuing from the mushroom body stalks in a plane just posterior 

 to that of the central body roots, continue backward and finally 

 enter the protocerebral core, and are the ''posterior roots" of 

 the mushroom bodies, as will be seen from the following descrip- 

 tion. The heavier stippling of the bundles of the posterior roots 

 represents, not larger nerve fibers, but larger bundles of fibers 

 within the main bundle. The stalks of the mushroom bodies have 

 nearly disappeared in this section, only the rtiost distal portion 

 remaining. 



It is therefore evident from figures 2 and 3, that in Bombus, 

 as in the ants previously described, the mushroom body stalks, or 

 the "inner roots" of various authors, do not end abruptly beneath 

 the central body as heretofore stated, but that their fibers divide 

 into two bundles, namely: the central body roots, figure 2, c.b.r., 

 and the posterior roots of the mushroom bodies, figure 3, p.r., 

 which go respectively to the central body and to the posterior 

 part of the protocerebral core. 



