MUSEUM OF COMPAEATIVE ZOOLOGY. 151 



preserved material ; the true nature of the cells, as well as the process 

 of deposition of the pigment, can clearly be understood from a section 

 such as is shown in Figure 5. The cutis contains here a group of irregu- 

 lar amoeboid (?) cells, distinguishable from the surrounding mass by their 

 refractive power, and containing from five to many deeply stained nuclei 

 3 /jl in diameter. The cells are all without any proper membrane, though 

 often surrounded by an envelope of connective fibres, and enclose a 

 varying number of highly refractive granules distinguished by indiffer- 

 ence to any coloring matter but picric acid, which they take up with great 

 avidity. Their natural color by transmitted light is a greenish yellow ; 

 by reflected, however, a dull brown or yellow. That the process of for- 

 mation is gradual becomes evident on the examination of a section like 

 Figure 5. In some cells are seen only a few such granules, or they are 

 confined to one part of the cell ; and all stages are present from this up 

 to a mass of closely packed granules in which neither cell plasma nor 

 nuclei are visible. Even in such cells the nuclei could be demonstrated 

 by prolonged staining and thin sectioning. The plasma of these cells 

 shows at first some slight affinity for hcematoxylin, which disappears as 

 the granules become more crowded. In the first stages of deposition 

 the granules are mere bright dots too small to be measured ; in the more 

 thickly crowded cells they have reached often twice or thrice the size of 

 a nucleus, and alongside of these are also granules as minute as those of 

 the earlier stage. Such cells are present not only in the cutis, but also 

 in all other organs of the body. They are not always as numerous as 

 shown in Figure 5 ; in the tentacles they are quite rare, whereas the 

 nervous system contains especially large numbers in all its parts. Some- 

 what similar cells were found by Burger ('90) in the nervous system of 

 .Nemertines. "Wherever these cells are found in Sipunculus they dis- 

 play the same structure, except that elsewhere than in the cutis they 

 are only found well filled with granules. Whether a migration actually 

 takes place, as is suggested by their evidently amoeboid character, I was 

 unable to determine. It is to the presence of large numbers of these 

 cells that the papillae of the posterior zone and the walls of the cerebral 

 canal owe their dark color. The pigment cells are present in much 

 greater numbers in large than in small specimens, i. e. in older than in 

 less mature ones. I can confirm the statement of Vogt und Yung ('88, 

 p. 38G) that fasting rapidly decreases their number. It is not a neces- 

 sary conclusion that this is to be regarded as reserve material. For 

 even waste may, under the pressure of failure in the food supply, be 

 drawn into the system and worked over again. 



