STRUCTURE OF THE PHARYNGEAL BARS OF AMPHIOXUS. 7 



cells of the sides of the bar. In the tongue bar these cells are 

 all of one kind (fig. 18), whereas in the primary bar two kinds 

 of cell (fig. 17, a, b) are present, one (a) being vacuolated, the 

 other (b) more granular. Langerhans noted this fact, though 

 he exaggerated the difference. So much for the epithelium of 

 the bar. I shall point out later how far these statements of 

 fact differ from those of my predecessors. 1 



Turning now to the cutis of the bar, i.e. the septal mem- 

 brane and the chitinoid rod: the septal membrane forms 

 a very thin sheet of tissue traversing the greater part of the 

 long axis of the section, and separating the epithelium of the 

 two sides. At the base of the epithelium of the Inner or 

 pharyngeal end of the bar the septal membrane splits 

 into two, and each of the two branches curves outwards to- 

 wards the surface ; this forking of the membrane leaves a 

 V-shaped space, which is converted into a triangle by a mem- 

 brane (cutis) at the base of the pharyngeal epithelium. 



In this triangular space is a blood-vessel, as Spengel and 

 Boveri have already described (figs. 7, 8). It may be called the 

 internal or Visceral vessel. At the opposite end the septal mem- 

 brane similarly divides into two, each half of which appears to 

 be continuous with the corresponding side of the rod (fig. 1). 

 This space, which differs somewhat in shape according to the 

 shape of the rod, but which is, on the whole, triangular, also 

 contains a blood-vessel the external, or somatic vessel. This 

 vessel was observed by Lankester (fig. 6), but overlooked by 

 Spengel (fig. 7), although he represents the space here, whilst 

 Boveri described it as existing only in the primary bar. From 

 the fact that, at each extremity, this septal membrane forks, 

 and from theoretical considerations, I believed this membrane 

 to be in reality double, as indeed it is represented by Stieda's 

 figure (PI. 6, fig. 3). But I was for a long time unable to 

 assure myself of this fact. I was unable to satisfy myself as to 

 the presence of two membranes here, for, owing to the refran- 

 gibility of the structure, it is difficult to make certain whether 



1 I have not observed the " muscle-cells " in the bar described by Rohon 

 and by Langerhans, J. Miiller and Schneider. 



