TISSUES OF LIGHT-PRODUCTION 



135 



a number of regions where the light may be seen. A vertical section of 

 one of these areas from the belly shows the epithelial structure common 

 to most selachian fishes, a moderately thin stratified layer with large 

 mucous cells showing in its outer part and a few widely spaced spines. 

 The large hollow mucous cells usually contain round homogeneous con- 

 cretions. The luminous organs are found lying between the spines 

 and are much more numerous than the spines are. They can be picked 

 out instantly as thickened regions of the epithelium into which branching 

 pigment cells have wandered. In this region the basal layer of the epithe- 

 lium is invagi- 

 nated into the 

 cutis as a pocket 

 of simple shape 

 with an opening 

 that is not con- 

 stricted. 



The six or 

 eight cells which 

 occupy the bot- 

 tom of this 

 pocket are en- 

 larged and their 

 distal ends are 

 collected into a 

 central mass. 

 Since the distal 

 end of each one 

 is filled by sev- 

 eral photochon- 

 dria or masses 

 of light material 

 secreted by the 

 cells, this mass 



is the point from FIG. 123. A light-organ from the skin of Spinax niger. b.m., base- 

 whirh the ' merit membrane of invaginated basal epithelium; lum.c., luminous 



art. t-K&f- 



Pff.c. 



lum. c. 



cells; I.e., lens cells with intracellular lens secretions; pg.c., pigment 

 emanates, and cells; lum.sec., luminous secretion ; art., artifact. X 580. 



the cells are the 



specific photogenetic cells of the tissue. Such cells of the same basal 

 layer as are found on the sides of the opening are not specialized, 

 and those of their proliferated descendants which stretch across the 

 opening of the invagination are only somewhat flattened and made 

 transparent to permit of the light's free exit. In the specimen from 

 which the drawing was made the poor alcohol fixation had caused 



