6 W. BLAXLAND BENHAM. 



Here, again, these rows of nuclei do not represent as many 

 layers of cells, for there is but a single layer of very long and 

 very narrow cells, with the nuclei at different levels in neigh- 

 bouring cells. In some of my preparations these cells are more 

 or less macerated, so that I was able to confirm the description 

 of them given by Langerhans (see fig. 19). Each cell, or at 

 any rate most cells for it is impossible to be absolutely certain 

 that all the cells reach the surface, though I believe this to be 

 the case carries a single very long cilium, and presents a 

 finely striated border. The nuclei of the lowermost row ap- 

 pear to be slightly longer than those of other rows, and are 

 close to the septal membrane or cutis, along which they form 

 a very well-marked series. Towards the extremity of the side, 

 both outwards and inwards, i. e. towards the atrial end and the 

 pharyngeal end, the lowest row of nuclei curve upwards to- 

 wards the surface, so that the epithelial cells become shorter 

 and shorter, and the number of rows less and less, till finally 

 there is but a single nucleus contained in a cell not much longer 

 than itself (see fig. 1). At the inner end of the bar the thin 

 membrane-like cutis curves outwards towards the surface, and 

 naturally the row of nuclei take on this curvature. At the 

 opposite end of the bar the row of nuclei follow the curve of 

 the chitinoid rod; but at this end, for a short space, the epi- 

 thelium is overlapped, as it were, by a row of three or four 

 cubical cells containing pigment, and not carrying cilia. These 

 cells are part of the atrial epithelium, the invaginated epiblast 

 of the pharyngeal slits. These pigmented cells differ from 

 those hitherto mentioned in containing circular (spherical) 

 nuclei, and herein agree with the cells constituting the epithe- 

 lium of the Outer end of the bar. 



This atrial epithelium is characterised by the larger size of 

 its cells, the absence of cilia, and the presence of relatively 

 small round nuclei, which are placed at different levels in the 

 cells (fig. 1). These cells are highest in the middle of this end 

 i. e. at the end of the long axis of the section through the bar 

 and decrease in length at each side of this point till they 

 graduate into the cubical pigmented cells which overlap the 



