528 CAROLINE BURLING THOMPSON 



13, 14 and 17, a.cm.). The second, here termed the ''posterior 

 dorsal commissure," arises several sections posterior to the ter- 

 mination of the first, and lies behind the central body (figs. 15, 

 20, 21, 22 and 23, vxm.). 



The anterior dorsal commissure 



This commissure was termed by Viallanes, "la commissure 

 cerebrale superieure, " by Kenyon ('96), "the superior dorso- 

 cerebral commissure," by Jonescu, "die dorsale Kommissur." 

 It is of considerable thickness in an antero-posterior direction, 

 being present in the brain of the Lasius queen in ten sections 

 each 6 ^ thick. It arises from the protocerebral fibrous core 

 lateral to the stalks of the mushroom bodies, and as it curves in 

 front of them there is doubtless an interchange of fibers. The con- 

 nection of this commissure with tract 6 of the mushroom bodies 

 is shown in figures 12, and 32 to 4D. Although the anterior dorsal 

 commissure is here described as a unit, careful examination of 

 its structure shows that it is composed of several fiber bundles, 

 or commissures, of different origin, and must be therefore a 

 compound structure. It seems probable that this is an indi- 

 cation of the ladder-like arrangement of fibers seen in many in- 

 vertebrate brains. 



The posterior dorsal commissure 



This commissure arises from the dorsal surface of the fibrous 

 core of the posterior portion of the protocerebral lobes (fig. 15, p.cm., 

 and figs. 20-23). The fibers curve upward from the lateral por- 

 tions of the lobes, then across, and down again into the core of 

 the opposite side. This commissure has a very characteristic 

 form, like a broad inverted letter PI, and is easily recognized. 

 The so-called "tubercles of the central body" often lie beneath it. 



This comiTiissure has been termed by Viallanes, "le pont des 

 lobes protocerebraux," by Kenyon, "the fibrillar arch," and by 

 Jonescu and also by Pietschker, "die Ocellarnervenbriicke." 

 Neither Viallanes' nor Kenyon's terms differentiate between this 

 second, posterior commissure, and the first, anterior commissure, 

 so they have not been used in this paper; Jonescu's term, "Ocel- 



