Protocerebrum of Micropteryx. 139 



is a thin syncytium, and hsneatli it are found the ganghon 

 cells and the axonic parts of the nervous system. Over the 

 protocerebrum the layer of ganglion cells is deep, and four 

 types can be distinguished : the normal type, the mushroom 

 body cells, the cells of the optic lobes, and the giant cells. 

 Neuroglia cells are found in the substance of the protocere- 

 brum in small numbers. The tracheal system of the 

 brain is very slightly developed. The j)rotocerebral lobes 

 are large, and in volume greatly exceed the other parts 

 of the protocerebrum together. The various parts of the 

 protocerebral lobes which have been described in other 

 insects are all present, though Micrajjteryx presents some 

 peculiarities, for the ventrolateral lobe and the middle lobe 

 are each divided into anterior and posterior portions. A 

 mid-dorsal lobe is also present, and to this I have given the 

 name tumulus, an organ which has not been described 

 before. The mushroom bodies are of a small, simple type, 

 and only one is developed on each side : the head is remark- 

 able because of the shape, ivhich is that of a rough sphere, 

 without any approach to the formation of a cuj). In section 

 it is seen to contain minute glomerular masses of nerve 

 fibres, which are regarded as association centres : these 

 are comparable to similar structures described in the mush- 

 room bodies of many insects, and also in antennary lobes 

 and central bodies. The origin of the stem is below, not 

 mthin, the head of the mushroom body, and it runs down- 

 wards and forwards in a definite space; it is rod-like, 

 and not perforated by a canal. The stem divides below 

 in a complicated manner which does not lend itself to 

 summarisation. I have suggested several possible homo- 

 logies for the parts into which the stem divides, and 

 my own view is that there are three roots to the mushroom 

 body in this insect — an inner, a forward, and a backward — 

 and that this is the typical number for the insect brain : 

 other views are also discussed. I have also given what I 

 believe to be the normal relations and characters of 

 these roots ; and I believe that this part, at any rate, of 

 my paper has some permanent value. The central body 

 is large, and consists of two capsules, as usual ; the outer 

 is the larger. There is no tendency towards the division 

 of either capsule in a fanlike manner, but the inner capsule 

 contains a number of minute glomerular bodies. The tracts 

 passing from or to the central body are numerous and some 

 of them are large. The nerves from the ocelli run inwards 



