PIGMENT TISSUES 



275 



thelial cells from the ex- 

 ternal covering of the 

 mantle- fold edge of a 

 mussel, Mytillus. 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 FlG - 2 3- Pigmented e P i- 

 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 

 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 





FlG. 239. Pigment cell from the peritoneum of the 

 sa.nd-la.unce A mmodytes. Fully extended. Arms can 

 be retracted until the cell is irregularly round. Two 

 nuclei, between which the non-pigmented centro- 

 sphere appears. X 180. 



