ip8 PATTEN. " [Vol. I. 



nerve, at the distal end of which it terminates, just beneath the 

 lower edge of the ommateum. 



Cross-sections through the middle of the inner wall show the 

 remnants of the ridge, or fold, on its outer edge, and the bundle 

 of nerve-fibres from the outer medulla, which penetrate the wall 

 just below the ridge (Fig. 6). A considerable space has been 

 formed between the outer wall and the inner surface of the 

 outer medulla. In median cross-sections this space opens out- 

 ward ; on either side of the median line, however, it is closed. 

 The cavity is nearly V-shaped, with the opening at the apex of 

 the V, on the front side of the brain. 



There is a thick layer of densely packed nerve-fibres continuous 

 with the medullary substance of the brain, and running under 

 the inner wall to mingle with the fibres of the outer medulla 

 (Fig. 6, n.f^). These fibres develop into the inner medulla 

 (Fig. 7, i.md.). 



The cells of the inner and outer walls are characterized, up to 

 a late period, by their deeply stained and turbid proto- 

 plasm. They are closely packed in a single layer, and 

 stand out in strong contrast with the graceful outlines of the 

 beautifully arranged and delicately stained ganglion-cells of 

 the middle wall. Toward the inner edge of the middle wall 

 there are some very large, either pear-shaped or conical, gan- 

 glion-cells, containing deeply-stained granules surrounding one 

 or two distally placed nuclei. These cells are unipolar gangli- 

 on-cells, although it is possible that some of the conical 

 ones are tripolar, for I have occasionally seen indications of two 

 prolongations of the broad distal ends of such cells. It is cer- 

 tain that some fibres, in the lo mm. stage at least, connect 

 the peripheral layer of the optic ganglion with the adjacent 

 neurilemma. 



During this stage the nucleated, heretofore irregularly ar- 

 ranged nerve-fibres connecting the outer medulla with the 

 ommateum undergo a remarkable change, i-n that they now 

 issue from the apex of the medulla as perfectly straight, 

 radiating fibres, which run against the neurilemma and are then 

 deflected toward the lower edge of the ommateum as a confused 

 mass of wavy fibres, which finally pass from there upwards 

 bet\veen the ommateum and the neurilemma, distributing on 

 the way small bundles of nerves to each of the minute om- 



