Sorensen, Study of Epiphysis and Roof of Diencephalon. 5 1 



enter the retina and connect with its sensitive elements (rods). 

 Besides the nerve-fibres, there are in this nerve scattered cells — 

 the remnants of the primitive cells of which it was composed. 

 These cells are also found in adult fishes. In the section studied 

 some occur near the middle of the nerve, some at the periphery. 

 A little bundle of filaments separates from the nerve, runs near 

 to it and then reunites with it. This was found quite highly- 

 developed in a series of sections across the pineal organ of 

 Petromyzon planeri. The nerve there was separated into three 

 larger and three smaller, nearly cylindrical and mutually parallel 

 parts. The beginning of the separation was not observed, but 

 the union occurred quite near its entrance into the parietal organ. 

 All the branches of this plexus attest a nervous character by 

 their fibrous texture. Besides this, each branch is separately 

 covered by an envelope of tissue, deeply colored, passing to 

 the envelope of the organ. No common envelope to these 

 nerves has been found. It is impossible to say what this separ- 

 ation might signify. 



The Pellucida. The histological composition is very sim- 

 ple. In the embryos and in young larvae it is composed of a 

 layer of cubical cells. Later these cells become cylindrical. 

 Their conformation is indistinct so that nuclei alone are seen 

 placed upon the border and turned toward the opening of the 

 vesicle. Further structure of the pellucida is generally indistinct 

 in adult fishes. It has sometimes the character of connective 

 tissue. In some places nodose thickenings were found inclined 

 toward the opening of the organ ; they are formed of elongated 

 cells. Sometimes the entire central portion of the pellucida is 

 thickened. The pellucida is in continuous connection with the 

 lateral walls of the organ. Pigment was found in the pellucida, 

 but solely in little groups of cells. 



The Retina. In the superior wall of the pineal organ of 

 embryos 5 mm. long, two kinds of cells were found, ordinary 

 cells, having the form of embryonic cells, and among them 

 others which have a more intense color and a more distinct con- 

 tour. The form of the last is irregular but they are always 

 elongated and placed vertically to the roof. Some of the cells 



