380 ON THE ANATOMY OF AMPHIOXUS LANCE OLATUS. 



sufficient magnifying power, it is seen to be composed entirely 

 of nucleated cells, very loosely attached to one another, but 

 enclosed in an excessively delicate covering of pia mater. The 

 cells are not arranged in any definite direction, except in the 

 middle third of the cord, where they assume a longitudinal 

 linear direction, but without altering their primitive spherical 

 form. The black pigment, formerly mentioned as existing 

 more particularly on the upper surface and groove, is observed 

 to be more abundant opposite the origin of the nerves ; and, 

 as it is regularly arranged in this manner in dark masses along 

 the anterior and posterior thirds of the cord, the organ in these 

 places, on superficial inspection, resembles much the abdominal 

 ganglionic cord of an annulose animal. Along the middle 

 third the pigment is not so regular, but appears in spots at 

 short intervals. When any portion of the cord, however, is 

 slightly compressed, and microscopically examined, it becomes 

 evident that there is, along the groove and mesial line of its 

 upper surface, a band, consisting of cells of a larger size than 

 those composing the rest of the organ. Some of these cells 

 only are filled with black pigment, but all of them contain a 

 fluid of a brown tint, which renders the tract of large cells 

 distinctly visible. When the compression is increased the 

 cells burst ; and the fluid which flows from the central tract 

 is seen to, contain jet black granules, which may be detected 

 as they escape from the cells. 



The nerves consist of primitive fibres, of a cylindrical 

 shape, with faint longitudinal stria?. The primitive fibres of 

 a trunk pass off into a branch, in the usual way, without 

 dividing ; and, where the trunks join the spinal cord, the 

 primitive fibres are seen to approach close to it, but without 

 passing into it. The greater part of the slightly protuberant 

 origin consisting of the nucleated cells of the cord, with a few 

 pigment cells interspersed, the exact mode of termination of 

 the central extremities of the primitive nervous fibres could 

 not be detected. 



