6 ARTHUR DENDY. 



the vesicle gradually passes into the pineal stallc. The upper 

 and outer wall of the vesicle is formed by the unpig-mented, 

 transparent "pellucida/' while the lower and inner wall is 

 formed by the " retina," under which term we may include 

 both the retinal epithelium and the layer of ganglion cells 

 and nerve fibres which underlies it (fig. 7). The retinal epi- 

 thelium has a characteristic opaque white appearance owing 

 to the abundant granules of white pigment imbedded in the 

 pigment cells, and this, seen through the transparent pellucida, 

 gives the whole organ its characteristic chalky appearance 

 even when seen with the naked eye or under a simple lens. 

 The line of junction of the pellucida with the retina, all i-ound 

 the circular margin of the upper surface of the organ, is very 

 sharply defined ; the wall of the optic vesicle is here thinner 

 than in any other part, and the edge of the pigmenteil retinal 

 epithelium appears from above as a distinct, opaque, white 

 margin to the pellucida (fig. 4). 



The thickest part of the wall of the optic vesicle lies pos- 

 teriorly just where it joins the stalk. The whole organ is 

 doubtless, as in Petromyzon, developed from the enlarged 

 distal extremity of a hollow pineal outgrowth, the proximal 

 portion (stalk) of which becomes solid and gives rise to the 

 pineal nerve. The original cavity of this outgrowth persists 

 distally as the cavity of the optic vesicle, and a smaller 

 portion of it persists in the thickness of the wall of the optic 

 vesicle, just in front of the point of entrance of the pineal 

 nerve, and forms the "atrium" of Studnicka. In Petro- 

 myzon, according to this author, the atrium communicates 

 freely with the main cavity of the optic vesicle, but yet shows 

 a tendency by enlargement to form an independent cavity. 

 In Geotria I have not been able to detect any communica- 

 tion between the atrium and the main cavity of the optic 

 vesicle; they appear to be completely separated from one 

 another. The atrium (fig. 7 At.) usually appears both in 

 longitudinal and transverse sections as a small oval or 

 almost circular cavity lined by columnar cells. In one series 

 of longitudinal section there are indications of one or two 



