54 Journal of Comparative Neurology. 



terior there is a layer of filaments, as in the case of Ammoccetes, 

 in connection with the brain by nervous filaments of the stalk 

 of the organ. In the layer of basic cells are scattered gangli- 

 onic cells which are more numerous at the extremity of the 

 retina. Beard calls this the ganglionic layer. They can be 

 considered as nervous from their form and susceptibility to stains. 

 They are found in small cavities formed by a fine membrane 

 (lymphatic cavities ?). Some prolongations appear to penetrate 

 into the layer of nervous filaments. A single prolongation up- 

 ward has been found. The third layer (nuclear layer of Beard) 

 is composed of nuclei belonging to different cells whose shape 

 it is impossible to follow. They are probably nutritive cells. 

 The exterior layer is composed of perfect and equal cylindrical 

 cells. These cells have the same significance as the sensitive 

 cells in Ammoccetes. They have above the large end a hyaline 

 point, or perhaps they form the delicate framework of the coag- 

 ulum (?) of the hyaline body. They narrow at the end, while 

 they enlarge where they contain the nucleus. Finally they pass 

 into an arched nervous filament which is directed toward the 

 base and backward toward the nervous layer. In the case of 

 adults this union with the nervous layer has not been noticed, 

 but we may suppose it. The union of several nervous processes 

 into a single bundle, such as Owsjannikow figures, has nowhere 

 been found. The apparent union is an entirely different thing. 

 These cells, moreover, in Petromyzon differ in thickness from 

 the sensitive cells (rods) of Ammoccetes. In Ammocoetes they 

 have the form ot thin rods, while in the case of Petromyzon 

 they are sometimes pyriform. This thickening is probably due 

 to pigment which penetrates their plasm and fills them, whereas 

 in Ammoccetes the pigment is never present. They are easily 

 stained, but only in their inferior part, where they contain little 

 pigment. 



Pigment. In the retina of the pineal organ of Petromyzon 

 the pigment is of great importance. It is always white, the 

 black being found only occasionally in the envelopes and in 

 small quantities. Seen in the microscope the white pigment is 

 quite black when viewed by transmitted light and in this case it 



