140 HISTOLOGY 



and make visible weak rays of light by reflection and, perhaps, by con- 

 densation, but which could not produce light. This same condition 

 holds true of the eyes of many animals. 



Technic. The tissues are fairly easy to section and stain, with the 

 exception of those that are covered with a hard chitin, which must be 

 removed. It is best to embed, and after scraping down to the chitin, to 

 remove it while the specimen is in the block and then re -embed. The 

 study of some of the details has been helped by special methods. The 

 ultimate branches of the air passages have been brought out by the use 

 of osmic acid on the living insect. This has entered the tracheae and 

 blackened the tracheoles to the exclusion of all but the most immediately 

 surrounding tissues. Also many of these tissues have been worked out 

 from crude alcoholic material, owing to the rarity of some of the speci- 

 mens. Flemming's fluid is probably the best general fixative for this 

 class of tissue. 



LITERATURE 



BURKHARDT, R. " Luminous Organs of Selachian Fishes," Ann. and Mag. Nat. Hist., 



7th series, Vol. VI, 1900. 

 JOHANN, LEOPOLD. " tiber eigentumliche epithelial Gebilde (Lichtorgane bei Spinax 



niger}," Zeits. f. Wiss. Zool., Band LXVI, 1899. 

 GREENE, C. W. "Light-organs of the fish, Porichthys. Histogenesis," Journ. Morph., 



Vol. XV. 



WATASE, S. " Animal Luminosity," Biol. Led. Woods Holl, 1898. 

 BRAUER, A. " Uber die Leuchtorgane der Knochenfische," Verh. deutsch. Zool. GeselL, 



Band XIV, S. 16, 1904. 

 CHUN, C. " IJber Leurhtorgane und Augen von Tiefsee-Cephalopoden," Verh. deutsch. 



Zool. GeselL, Band XIII, S. 67-91. 



