Protocerebrum of Micropteryx. 119 



" normal " ganglion cells. They stain deeply with hsema- 

 toxylin. The neurilemma is continuous over the whole 

 brain dorsally and ventrally and also over the optic lobes ; 

 it is continued downwards to cover the ventral parts of the 

 central nervous system. 



The Ganglion Cells. — These are spread over the whole 

 anterior and superior parts of the brain, in a layer which 

 reaches its greatest thickness mid-dorsally. The layer is 

 discontinuous or absent beneath the protocerebrum. On 

 the upper side of the brain the cells may be as many as 

 15 cells deep, particularly near the middle line between 

 the ocelli. Various types of ganglion cell must be distin- 

 guished. They all possess a spherical nucleus and a very 

 small quantity of cytoplasm. The normal cells {g. c.) 

 cover the protocerebrum above, before and behind. Cells 

 of this type, which is much the most abundant, are either 

 motor or else cells of connecting-fibres (Kenyon). The 

 cells of the mushroom body {mh.c.) are found as a rounded 

 mass of cells lying just over the head of that organ. Their 

 nuclei stain heavily with hsematoxylin ; they are also 

 smaller than the normal cells. The fibres from these cells 

 pass into the mushroom body. The distinction in size 

 between these cells and those of the normal type is not very 

 great ; it can be best observed in material fixed in Gilson's 

 fluid. The cells of the optic lobes {o.c.) are still smaller 

 than those of the mushroom body; and their nuclei 

 are absolutely spherical and stain very heavily and com- 

 pletely. No structure within the nucleus can be detected 

 in ordinary sections and this gives to the masses of cells a 

 very characteristic appearance. The cytoplasm, per contra, 

 is scarcely stained at all. These cells form a deep coating 

 which completely envelopes the three optic gangUa ; this 

 layer is less deep above than below. In Micropteryx giant 

 cells [gi. c.) are found in small numbers round the base of 

 the mushroom body just where it passes into the proto- 

 cerebral lobes (fig. 12, etc.). These cells are few in number, 

 hardly more than a score on each side. Their nviclei are 

 spherical and about four times as large as those of normal 

 cells ; there is a considerable quantity of cytoplasm, which 

 can be stained with eosin ; this distinguishes it at once 

 from the cytoplasm of the other types of ganglion cell. 

 The nucleolus is generally clearly seen. Haller states that 

 these cells are mainly, but not entirely, concerned with 

 conduction across the middle line, and that their axons 



