272 



HISTOLOGY 



pigment cells divide mitotically. Such mitosis has been observed by 

 Flemming and Zimmermann (Fig. 234). 



As an example of diffused pigment in an animal product we may 

 take the secretion of the ink-sac of a squid. The ink-sac is a pear- 

 shaped organ lying beneath the integument of the mantle wall. In 

 origin it is an invagination of the walls of the rectum. Its size varies 

 with the size of the animal ; its greatest transverse dimension is about 

 one eighth the diameter of the body at the level of the ink-sac. From 

 the epithelium of the fundus of this sac, epithelial pouches are constricted. 



FIG. 235. A, diagram of the arrangement of lobules in the ink-sac of a squid, Loligo Pealii. 

 x, regions of lobule generation; d, regions of discharge of ink from lobules; B, epithelium of 

 youngest lobules, no ink apparent ; C, older epithelium, first appearance of ink in the cyto- 

 plasm; D, epithelium from still older lobule; much ink formed and stored in lobule; E, more 

 ink being produced by cells and ready for discharge. A, X 20. B-E,X 600. 



These, as they are crowded toward the neck of the sac by the continued 

 proliferation of glandular vesicles, increase in size and finally rupture 

 in the vicinity of the neck of the sac (Fig. 235, 4). The pouch, as it 

 leaves the fundus, is lined with columnar epithelial cells, with oval nuclei. 

 The cytoplasm is homogeneous and free from any pigment (Fig. 235, B). 

 As the glandular pouch increases in size, the cells become higher and 

 greater in diameter. At the distal ends of the cells, conspicuous pig- 

 mented drops appear (Fig. 235, C). As the glandular vesicle con- 

 tinues to grow, the cells become wider, their nuclei become more rounded, 

 while pigment is elaborated at the expense of the cytoplasm. The cells 

 disintegrate distally and thus pour out into the lumen of the vesicle 



