1^8 BIOLOGICAL LECTURES. 



into a pavement epithelium. The inside of each wing-bag is 

 hollow and contains tracheae, blood cells, and fat cells. At 

 first all of the cells composing the wing-bag are similar, each 

 to each, but soon we notice that at regular intervals some of 

 the cells are becoming larger than the others and are begin- 

 ning to send out each one a process, which extends outward 

 beyond the general level of the wing surface. These cells 

 are destined to form the scales. The process which arises 

 from each scale-forming cell becomes larger and flattens out 

 into a spatula shape which exactly resembles that of the future 

 scale. Then the cell secretes a layer of chitin over its outer 

 surface, and thus the scales are formed. The scales are now 

 completely filled with protoplasm and are as transparent as 

 glass ; but soon the protoplasm begins to shrink away, leav- 

 ing them little hollow, chitinous bags, which diffract the light 

 and are therefore white in color. Soon after the scales have 

 become hollow the blood or haemolymph of the pupa enters 

 them, and they become dull yellow in color ; and then the 

 mature color begins to appear, and gradually deepens until it 

 acquires the tint of the imago. The pigment, then, is formed 

 within the scales during the time when they are filled with 

 h^molymph, and the natural inference is that the color is 

 derived from the haemolymph, or from some of its constitu- 

 ents, by unknown chemical processes. It is interesting to 

 notice that Friedmann ('99) finds that in Vanessa urticae the 

 scale-forming cells, and hypodermal cells, and also the blood 

 cells within the haemolymph, are filled with small fatty gran- 

 ules which stain deeply in Hermann's fluid. These fatty gran- 

 ules enter the scales and contribute toward the formation of 

 the color. No such granules were observed by me in 1896, 

 when studying the scales of Danais plexippus and other forms ; 

 but all of my material was killed in Perenyi's fluid or in corro- 

 sive sublimate, and Friedmann observes that corrosive sublimate 

 fails to demonstrate the fatty globules. It is probable, there- 

 fore, that my observations are wrong in this respect. 



If it be true that the colors of the mature wing are derived, 

 by various chemical processes, from the h^molymph, or from 

 some of its constituents, then one ought to be able to manu- 



