404 Mr. W. J. Sollas on the 



The Chones (PI. XVII. figs. 1, 3). — These, as well as the 

 whole of the canal-system, with the exception of the ciliated 

 chambers, are lined by a delicate epithelial layer. 



Their walls are chiefly composed of concentrically arranged 

 fusiform fibres, very similar to muscle -fibre, but staining much 

 less intensely with carmine. Near the lower end of the chone 

 this layer becomes continuous with the thick conical muscle 

 which " plugs " the bottom of the chone and protrudes its 

 apex into the subjacent crypt. The mass of the muscle con- 

 sisls of true, fusiform, muscular fibres concentrically arranged 

 around a central canal, which is lined by epithelium and 

 associated sharp-rayed stellates. The subcortical layer, where 

 it joins the muscle, frequently dovetails with it, thrusting a 

 small wedge of gelatinous connective tissue (c) into its side 

 and receiving on its lower face a short superficial extension of 

 the muscle-fibres, while its upper strictly fibrous portion (f) 

 passes gradually into the muscle, the muscle having very 

 much the appearance of being an over-development of the 

 subcortical fibrous layer : this appearance is probably very 

 near the truth, both structures having most likely been de- 

 rived from a primitively indifferent fibrous layer, which on the 

 one hand became modified into connective and on the other 

 into muscular fibres. The chones are clearly the modified 

 outermost vesicles of their associated canals, and their sphinc- 

 ters the modified ruga? of these canals. Hence the canal-walls 

 contribute a share to the formation of the fibrous layer of the 

 cortex. 



The Canals. — The ultimate ramifications of the canal-system , 

 as well as the smaller trunks into which they collect, are 

 simple excavations in the mark lined by epithelium, which 

 gives them, especially those having an incurrent function, a 

 very sharp and definite outline. In the case of the larger 

 canals the mark immediately surrounding them becomes a 

 little less granular than elsewhere, and stains a little less 

 deeply with carmine ; hyaline fusiform fibres and sometimes 

 granular fusiform cells appear in it, sometimes lying separate 

 from each other, sometimes accumulated side by side and 

 with overlapping ends forming a fibrous band. They are ar- 

 ranged both longitudinally and transversely with respect to 

 the axis of the canal ; but in the trabecular excurrent canals 

 their position is governed by that of the trabecule, which 

 they traverse more or less longitudinally. The structure of a 

 canal-wall when fully developed exhibits, in transverse section, 

 first, on the inside, a layer of epithelium, next a layer as much 

 as O00125 inch thick of fibrous tissue, and then a layer of 

 gelatinous connective tissue adjoining the mark. 



