STRUCTURE OF THE PHARYNGEAL BARS OF AMPHIOXUS. 21 



FIG. 17. Cells from the atrial epithelium of a primary bar, from a partially 

 macerated section (cochineal), a. Vacuolated cells. 6. More granular, 

 narrower cells, the nucleus being near the free end. c. A portion of a cell, 

 which is probably similar to d, with a narrow peripheral portion, and dilated 

 nuclear region below. 



FIG. 18. Three cells of the atrial epithelium of a tongue bar, from a 

 partially macerated section. 



FIG. 19. Cells from the lateral epithelium of a gill bar, from a partially 

 macerated section. The nuclei of the cells are seen to be at. different levels. 

 Each cell carries a single long cilium. The small cell to the left may be a portion 

 of a larger cell, or may be really a basal interstitial cell. 



FIG. 20. A small piece of the septal membrane, with a nucleus (#.) (from 

 a transverse section of a bar). The membrane appears to be double, and the 

 nucleus lies between the two sheets. 



FIG. 21. A portion of a longitudinal section of a bar, showing the septal 

 membrane and three elongated, compressed nuclei (.). Only a small por- 

 tion of the epithelium of the bar is filled in, in order to show the relative 

 size of the nuclei of the septal membrane and those of epithelium (ep.}. 

 (Hsematoxyliu. Under Zeiss's apochromatic 4 mm., aperture '95, with com- 

 pensating ocular 8.) 



FIGS. 22 26. Five consecutive sections to show the passage of a tongue 

 bar into the endostyle, and the communication of the rod cavity with the sub- 

 endostylar ccelom. 



Fig. 22. Transverse section of the endostyle and a neighbouring tongue 

 bar. The details as to arrangement of nuclei are only approximately 

 accurate. It is merely desired to exhibit, the general topographical 

 relations of the structures, end. Endostyle. end. sk. Subendostylar 

 skeleton. end. coel. Subendostylar ccelom. cce. ep. Nuclei of the 

 ccelomic epithelium, at. ep. Atrial epithelium, art. Subendostylar 

 artery. P. The lowermost part of a primary bar fused with the endo- 

 style. x. The peculiar tissue of elongated vacuolated cells (not 

 reticular tissue). T. T. Tongue bars. rod. The contained hollow rod ; 

 on the left the bar has already reached and fused with the subendo- 

 stylar tissue, on the right it is still free. 



Fig. 23. The next section. The rod of the tongue bar on the left, appears 

 to have a narrow cleft in its wall, so that its cavity communicates with 

 the Subendostylar coelom ; but this is not so distinct as in the case of 

 the right rod in Fig. 26. (The lithographer has exaggerated this cleft.) 



Fig. 24. The right portion only of the next and following sections is 

 drawn. Here the rod of the tongue bar (T.) is distinctly smaller, and 

 approaches the corner of the Subendostylar coelorn. 



