TISSUES OF LIGHT-PRODUCTION 



123 



cell to be oxidized by materials brought to it or by the free oxygen in 

 air and water. Wherever used, it only acts properly in an alkaline 

 medium. 



The cells that secrete luciferase are of many kinds. They may be 

 ectodermal or mesodermal in origin. In a few cases they are cells 

 that are used mainly for other purposes, and the light-production is a. 

 secondary matter. Examples of such are the muscle cells of Ophiura 

 and the blood cells of Hystrix. In the majority of cases, and always 

 where the organ is of high efficiency, the cells are devoted exclusively 

 to this function, although it can easily be seen that they originated from 

 some of the ordinary tissues of the body, as the fat body in insects, the 

 epithelial cells in fishes, etc. 



Cells modified to make light are, in the primitive forms, epithelial in 

 character. They appear as low or tall columnar cells. In the more 

 highly specialized forms, and especially in the forms of mesodermal origin, 

 they are grouped into a mass of polygonal cells. Where the cells are 

 columnar in shape the nucleus is found near the proximal end, as in other 

 cases of columnar gland cells, and the materials are absorbed through 

 the proximal surface and passed in the process of elaboration toward the 

 distal end, where they are either used to produce light in situ or dis- 

 charged to be used outside the cell. On the other hand, where the cells 

 are of the second or polygonal shape, the food materials are absorbed 

 from all surfaces and the luciferase is used in situ, the products of 

 combustion being passed out through the same surfaces into the blood. 



Sometimes these cells are found alone in an organism, but in the 

 majority of cases and almost always in the highly developed organs, they 

 are accompanied by one, two, or three other and accessory tissues; the 

 reflecting tissues, the lens tissues, and the pigment mantles. 



The reflecting tissues are of two forms : the connective-tissue reflectors 

 and the urate reflectors. The first of these is a very ordinary looking 

 fibrous or thin-layered connective tissue that cannot be told in any way 

 from a common lax or fibrous connective tissue except that it will reflect 

 the light most perfectly. It is developed from the same tissue that 

 forms the corium of the skin, and its nuclei are like those of other tissues 

 of the same kind. Its fibrils or plates are usually developed at right angles 

 to the direction of light emission. As far as can be seen, the refractive 

 power is due to the presence of innumerable and almost invisible particles 

 deposited in the substance of the reflecting tissue. No other tissue in 

 the body can reflect the light in this way except the pigment found on the 

 surface of the body in some fishes. 



The second kind of reflector is made of layers of large cubical cells 

 that have deposited in their interior, crystals of some urate that reflects 

 the light most perfectly. This form is found in the insects and perhaps 



