538 CAROLINE BURLING THOMPSON 



figure 27. It will be noted that the amount of cytoplasm is 

 very small in all the mushroom body cells. 



Group I (figs. 27, 29) forms a prominent oval mass of large 

 cells which are much larger than those of any other group. Both 

 cell body and nucleus are large, the nucleus is oval, with a large 

 nucleolus. The axons of these cells unite in a conspicuous bun- 

 dle, h, that runs downward in the center of the stalk (figs. 32 

 to 40). These efferent fibers give reason for regarding this cell 

 group as probably an important motor center. 



The Groups II are shaped like a fan or broad wedge, being 

 arranged in radiating rows of ten or twelve cells whose fibers 

 converge into definite and prominent bundles, g, also efferent in 

 nature. The two bundles gl. and gr. from each Group II pass 

 into the center of the stalk together with, but on the outside 

 of, the bundle h, from Group I. The cells of Group II (fig. 27), 

 are slightly elongated with oval nuclei and very little cytoplasm. 



Group III is usually distinctly separated from Group II by a 

 narrow space, or its outline may be determined merely by the 

 different size and arrangement of the cells. The surface toward 

 Group III may likewise be bounded by a slight space from which 

 cells are absent, or the two groups may merge into one without 

 any line of separation. The cells of Group III are very little 

 smaller than those of Group II, but it will be noted in figure 27 

 that there is less cytoplasm, that the cell bodies are not elongated 

 and that the nucleus is rounder. These cells are usually arranged 

 in vertical rows or layers, seven or eight cells deep. The fibers 

 from these cells run into the apices of the calyx, but the bundles 

 can rarely be traced very far. Tracts e, r, m, n, to be described 

 below, originate from the cells of Group III. 



Group IV forms the basal part of the cell envelope, and is 

 the smallest group, both in the size and in the number of the 

 cells. The cytoplasm forms merely a narrow rim around the 

 small round nucleus. 



These four zones of cells, seen in section as seven groups, are 

 present and typical in all three castes of the three genera under 

 discussion. Figure 27, from the worker of Lasius, and figure 29, 

 from the worker of Camponotus, show that the largest cells of 



