HARDWICKE'S SCIENCE-GOSSIP. 



37 



pigment granules were seen sometimes congregated 

 in a spheroidal mass round the nucleus, at other times 

 diffused in a radiating manner through the cell or 

 into the processes. The movements were accompanied 

 with shortening and elongation of the ramifications. 



The changes of colour observed in the animals no- 

 ticed previously are caused by alterations in the form 

 of the pigment-cells, and are produced either sponta- 

 neously, or by variations in the intensity of light, or 

 by other external stimuli. R. Wagner has observed 

 extraordinary mobility in the pigment-cells of cuttle- 

 fishes, which contain pigment granules of different 

 colours and are termed " chromatophores." Von 

 Wittich * has described the changes produced in the 

 cells of Hyla arborca by electrical excitation, although 

 Professor Rollett was unable to perceive any influence 

 exerted on the pigment-cells of batrachians by the 

 action of induction shocks of electricity. 



Fig. 36. — Cortical section of horsehair, showing the linear 

 arrangement of pigment, i-inch. 



The pigment granules vary in colour and shape. 

 Under the highest powers they exhibit no definite 

 form, being often subcylindrical, or elongated with 

 rounded extremities. Beale says : " They may be re- 

 moved from the cell, and when they escape into the 

 surrounding fluid they exhibit molecular movements." t 

 In vertebrate animals the pigment is derived from the 

 red blood corpuscles. These, as they grow old, part 

 with their colouring matter to the serum. From 

 thence it is taken up by the pigment-cells and con- 

 densed in their interior, where it undergoes several 

 chemical changes and passes through several shades 

 of colour. Rindfleisch states : " Should they (the 

 pigment granules) be numerous enough to fill the 

 protoplasm of a cell, the colourless nucleus is partly 

 pushed aside, partly surrounded ; the pigmented cell 

 appearing to be perforated by a circular gap or 

 hole. Flat cells (choroid coat of the eye), in which 

 the nucleus is in contact with both surfaces at 



* Miiller's _" Archiv," 1854, p. 41. 



f "The Microscope in Medicine," p. 154. 



once, retain their characteristic aspect (fig. 34). In 

 spheroidal cells, however, the nucleus ultimately 

 disappears, leaving a coloured corpuscle, in which 

 only the external form of the cell can still be recog- 

 nised." * 



The source of the pigment in the invertebrata is 

 not definitely known. 



Pigment is also found in animals deposited in the 

 hair, feathers, and other tegumentary appendages. 

 In these situations it is not enclosed in cells. The 

 pigment granules in the hair are located in the cortical 



Fig. 37. — Portion of 

 broken scale from 

 Argynnis Adififie, 

 i inch. 



Fig. 38. — Black scale from I'nnessa 

 urtiaz, i inch. 



tissue, disposed in lines running parallel to the axis 

 of the hair (fig. 36). They are exceedingly minute, 

 estimated in the human hair at j5$ m inch in dia- 

 meter. It is here that we must look for an analogy 

 with that occurring in Lepidoptera. The scales of 

 Lepidoptera are homologous to hair or feathers in their 

 situation and appearance, and also analogous in their 

 function. We find likewise a similarity between the 

 arrangement of pigment in the scales and that in hair. 

 The pigment is deposited between the fine membranous 

 layers which compose the scales, and is arranged in 

 parallel lines corresponding to the situation of the 

 ribs or striae. Under the microscope these appear as 

 straight dark lines with irregular edges (fig. 37). As 



* "Pathological Histology," vol. i. p. 62. 



