ON THE PHOTOGENIC ORGAN OF THE KNIGHT-FISH. 367 



They crowd so thickly that each stands almost vertically, and 

 they lie parallel to one another. Indeed, the tubules appear to 

 belong to a single gland with several large central spaces (Fig. 3, 

 pi. I.). 



Each photogenic organ of Monocentris is, however, a compound 

 gland made up of some nine components which unite secondarily 

 into a functional whole. The distal part of the gland still shows 

 the compound nature and this part consists of many narrow 

 canals, each representing the duct of an individual acinous 



, 



FIG. 2. Sub-mental photogenic gland of Monocentris japoniciis in longitudinal 

 section passing through its opening. About X 30. cap, capillaries; chr, chroma- 

 tophores; con, connective tissue; dp, dermal papillae; dt, emissory ducts; m, 

 muscles; rt, reservoir; op, openings of gland; os, sub-maxillary bone; se. t, secretory 

 tubules; tr, trabeculae. 



gland. The general constitution of the organ is represented in 

 Fig. 2. 



HISTOLOGY OF THE PHOTOGENIC ORGANS. 



Under a high magnification each tubule shows an epithelial 

 lining composed of cubical cells arranged in a single layer except 

 at the fundus region which is the seat of a most active secretion, 

 where the walls are considerably thickened with two or three 

 cell layers (see Fig. 4, pi. I.). The stratified aspect of this 



