152 Explanation of Plates. 



0. c. = cells of optic lobes. 

 oc. gl. = ocellaiy glomerulus. 

 oc. n. = ocellary nerve. 

 0. 1. = optic lobes. 



ou. ca. = outer capsule of central body. 

 pc. I. = protocerebral lobes. 



po. br. = posterior branch (= backward root) of mush- 

 room body. 

 s. n. = sensory antennary nerve. 

 St. = stem of mushroom body. 

 sw. hd. = swollen head of ascending branch. 

 siv. St. = swollen foot of stem. 

 tm. = tritocerebrum. 

 tr. = tracheal tubes. 

 tu. = tumulus. 



vl. I. = ventrolateral lobe of protocerebrum. 

 vl. I. a. and vl. I. p. = its anterior and posterior portions. 



Explanation of Plates. 



Plates VII to IX (figures 1 to 16) represent a series of vertical 

 transverse sections through the protocerebrum of Micwpteryx, 

 partially diagrammatic. Each figure represents a successive sec- 

 tion 3"5 IX thick, except that single sections are omitted between 

 figures 6 and 7, 9 and 10, 14 and 15, and 15 and 16. 



The neurilemma is shown as a black line, the cells of the mush- 

 room body {mh. c.) and those of the optic lobes (o. c.) by black masses, 

 and the normal ganglion cells {g. c.) by dark grey, except where one 

 or two occur alone, in which case they are represented as indi- 

 vidual black dots (e. g. fig. 5). Giant cells {gi. c.) are always drawn 

 individually. Axonic substance is shown pale grey, except the 

 stem of the mushroom body, which stains heavily and is coloured 

 dark, and certain other parts of the protocerebrum which are 

 shown in medium grey because they stain slightly more intensely 

 than other parts. Bands of fibres are shown as dark lines when 

 they are striking features of any section. 



The upi^er part of each figure is dorsal, the lower ventral. 



Plate X (fig. 17).- — This plate is from a thin section, hence the 

 amoimt of free space among the ganglion cells. The drawing re- 



