2 7 6 



HISTOLOGY 



have become modified. They are found, here, in the central area of 

 the cell which is now a syncytium, and this area, which was already 

 differentiated from the ectosarc, has become further separated by the 



formation of a distinct 



blc. x^^/y* -'v^rST^v . v-fc. -!^ sac about its content of 



pigment-bearing c y t o- 

 plasm. The sac-waU is 

 but partly formed in 

 Figure 242. 



The young, but com- 

 pleted, chromatophore is 

 seen in Figure 244. 

 Marked changes have 

 taken place. The pig- 

 ment sac is inclosed by a 

 tough, membranous wall. 

 But one nucleus remains 

 within the sac, and this 



FIG. 240. Three liver cells from the salamander Crypto- IS larger and is spetial- 

 branchus. They show, in A, B, and C, three successive ized to direct the Dip> 

 stages of pigmentation, bl.c., blood cell. 



ment-formmg activities 



of the inclosed cytoplasm. The other nuclei have multiplied by amitosis 

 and have migrated outside the sac, so that one lies in the basal portion 

 of each cell process. These nuclei control the myo-nbril-forming activ- 

 ities of the cytoplasm in the processes whereby they render them 

 contractile. The processes are now able to stretch the central pigment 

 sac out until it becomes very broad and visible, 

 allow it to contract by its elasticity 

 and become almost invisible. The 

 chromatophores are arranged in two 

 or more different sets, and all the 

 members of each set are connected 

 with a common nerve plexus. At 

 the same time the different sets are 

 independent of each other as to 

 nervous control, and so but one or 

 two or all or none of them may be FlG . 24I .- A , B , and c. Three youngest 



expanded at a time. As each Set stages in the development of a cephalo- 



contains a differently colored pig- 

 merit in its pigment sacs, we have 

 an explanation of the rapid changes 

 of color which pass over a squid or octopus at different times. 

 At one time it may be red, the next transparent, and presently turn 



pod's chromatophore. Each stage shows 

 a nucleus and centrosphere. The two 

 older ones have put out processes. (After 

 C. CHUN.) 



