312 DISCOVERY REPORTS 



though not in the vertical plane (Fig. i b,fib.). The fibrous mass tapers fairly regularly 

 around the periphery of the circular organ, but ceases abruptly at its margin, giving the 

 clearly circumscribed appearance noted by Dr Kemp. Its dimensions are, in the various 

 organs, of the order of 500/^ in diameter and 100 ju in thickness, and with Mallory's 

 triple stain it assumes a deep blue colour slightly tinged with purple. 



Embedded in the fibrous mass are numerous uniformly scattered small rounded 

 nuclei, about 5 /t in diameter and staining vividly with acid fuchsin. They are apparently 

 the nuclei of the fibre mother cells. In appearance they differ from the connective 

 tissue nuclei found elsewhere in the limb. Between the deeply staining fibrous mass and 

 the integument of the limb a delicate layer of fibres, staining lightly with aniline blue, 

 may be seen (Fig. i a, siip.fib.). This layer is readily distinguishable from the deeper 

 fibrous mass, but it is not clear whether it is to be regarded as having the same nature 

 and derived from it, or whether it is a connective tissue layer like that on the inner side 

 of the main fibrous mass. 



The organ is cut off from the haemocoelic space of the limb on its inner side by a 

 thin sheet of delicate connective tissue fibres, some of which are in connexion with the 

 network of fibres extending across the blood space. Laterally the connective tissue layer 

 extends beyond the organ as a thinner sheet on the inner side of the chitogenous 

 epithelium. 



The most remarkable feature of the organ, however, is the presence of rather ill- 

 defined rod-like bodies which stand perpendicularly to the surface of the organ and 

 occupy the centres of cylindrical spaces in the fibrous mass (Fig. i and Plate XXIV, 

 fig. I, phot.cen.). These rod-like bodies are rounded at their ends, and are about 15 /i long 

 and 3 /i in diameter. With Mallory's triple stain they assume a yellowish brown colour. 

 The cylindrical spaces in which they stand are otherwise filled with a finely punctate 

 substance, which may be a coagulation, staining faintly blue with Mallory. That these 

 spaces with their contained rods are the seat of light production (presuming the organ 

 to be truly a photophore) appears to be indicated by the absence, or at any rate the much 

 more feeble development, of the fibrous mass of the organ over their distal ends. This 

 is clearly seen in Fig. i . Such an arrangement would permit of the outward passage of 

 any luminescence generated within the "photogenic centres", as I have called these 

 structures. The granular appearance, in superficial view, of the photophores is clearly 

 caused by the absence, at intervals, of the mass of densely packed fibres forming its 

 main body. 



Lens-less organs, believed to be luminous, have been recorded in S. robustus and 

 related species (lUig, 1914; Hansen, 1919, 1922; Sund, 1920; Burkenroad, 1937). 



These observations have been confirmed by Welsh and Chace (1938, p. 371) who, 

 previous to obtaining histological preparations, express the view that the pigmented 

 structures observed by them are actually luminescent in life. It should be mentioned, 

 however, that Hansen (1922, pp. 97, 120) expresses some doubt that this is their function, 

 since they lack a well-defined lens. Later in this paper it will be shown that unquestion- 

 able photophores may be without a lens. 



