PIGMENT TISSUES 



275 



FIG. 238. Pigmented epi- 

 thelial cells from the ex- 

 ternal covering of the 

 mantle- fold edge of a 

 mussel, M ytillus. X 1500. 



The chromatophore of the squid is a complex structure composed of 

 a central pigment cell surrounded by smaller cells, which send their slen- 

 der bodies in a radial fashion from the pigment 

 cell to supporting connecting tissue. These cells 

 are contractile and respond to nervous impulse 

 brought by a special nerve supply. In this way 

 the shape of the chromatophore is modified. 

 Change of form alters the color of the chromato- 

 phore. Chun has shown that this complex chro- 

 matophore arises from a single embryonic con- 

 nective-tissue element, which is an oval cell as 

 represented in Figure 241, A. 



This cell is at first small and contains a single nu- 

 cleus. The nucleus is eccentric, being placed at the 

 extreme end of the cell. This is apparently so, 

 because of the presence of an astral figure which 

 probably represents a centrosome with its rays. 



As the cell grows, it may be noticed (Fig. 241, B] that the peripheral- 

 cytoplasm or ectosarc becomes differentiated from the remainder of 

 the cell by becoming clearer. It also sends out sharp-pointed pro- 

 cesses, which are well seen 

 in (C). These processes in- 

 crease much in length and 

 thickness, and when the cell 

 is much larger, some of them, 

 usually three or more on one 

 side, form a contact with 

 some nerve fibers which come 

 from a nerve plexus in the 

 integument. This condition 

 is shown in Figure 242. 



The next feature to de- 

 velop is a differentiation of 

 the structure to enable the 

 cell to perform its dual func- 

 tion. These two functions 

 are : the production and main- 

 tenance of a pigmented area, 

 and the mechanical operation 



FIG. 239. Pigment cell from the peritoneum of the 

 sa.nd-la.unce Ammodytes. Fully extended. Arms can 

 be retracted until the cell is irregularly round. Two 

 nuclei, between which the non-pigmented centro- 

 sphere appears. X 180. 



of that area by a muscular 

 apparatus. A multiplication 

 of the nucleus takes place ; in Figure 242 the single nucleus has already 

 divided into two. In Figure 243 there are four nuclei, and two of them 



