Protocerebrum of Micropteryx. 127 



the stem may be called the ascending trunk {as. tr.). The 

 homologies of this organ are obscure and will be discussed 

 later. It runs upwards for a very short distance and 

 divides into two portions. 



At its point of division it is swollen. For the moment 

 let us call one of its branches the ascending and the other 

 the posterior branch.* 



The ascending branch {as. br.) runs up directly to the 

 surface of the brain and is there swollen into an acorn- 

 shaped head. It is covered by a thin layer of ganglion cells 

 of the normal type, and these send their axons into the 

 branch. This is a point of interest, for here and also at 

 the foot of the stem we have a few nerve cells which appear 

 to belong to the mushroom body. Similar conditions have 

 been occasionally described in other insects, notably in 

 Periplaneta by Haller. 



The posterior branch {po. br.) of the ascending trunk runs 

 backwards and slightly upwards and inwards. Its ter- 

 minal part is bifid, but the two portions do not diverge from 

 one another. It is completely enclosed in the protocerebral 

 lobes, and even in fixed material is not always very easy 

 to see. It is about two-thirds the length of the stem. 

 Before we proceed to discuss the homologies of these organs 

 it is absolutely necessary to grasp their anatomy and 

 relationships. 



Let us now consider the homologies of these three 

 branches of the stem. The first I have already identi- 

 fied as the inner root {innere Wurzel of Kiihnle). This 

 identification rests on its relations to other organs. 



The ascending tnink is an organ for which I find no 

 parallel in any insect brain yet investigated. This is not 

 very remarkable when we remember that the brains of no 

 Lepidopteron or Trichopteron have yet been fully described. 

 According to a view which I now put forward the ascend- 

 ing trunk is to be regarded as the united base of the 

 forward root {vordere Wiirzel) and of the backward root 

 {riicMavfige Wurzel). Great obscurity exists with regard to 



* I use the term " trunk " and " branch " rather than " root " 

 in order not to commit myself to any view as to homologies which 

 are fully discussed later. The terms are of a provisional nature. 

 It may be objected that I am adding to the synonymy, but it is 

 almost a necessity to have some unequivocal name for an organ 

 until its homologies are fixed. The word " root " I use as a full 

 equivalent of the German Wurzel. 



