322 



Basic Structure of Vertebrates 



phores. Chromatophores are usually richly branched. The pigment 

 may at one time be distributed throughout the processes ("expanded" 

 phase), at another time densely massed in the central part of the cell 

 ("contracted" phase). Some pigment-cells are migratory. 



Collagenous fibers 



Connective tissue cell 



Elastic tissue fiber 



Fig. 264. Elastic tissue. Ligamentum nuchae of ox. The 

 elastic tissue fibers are broad and straight, the collagenous 

 fibers are in wavy bundles. Note the scarcity of connective- 

 tissue cell nuclei. (Courtesy, Bremer-Weatherford : "Text- 

 Book of Histology," Philadelphia, The Blakiston Com- 

 pany.) 



Skeletal Tissues 



Notochord. The essential material of the notochord consists of 

 cells each of which contains a relatively enormous vacuole occupied by 

 a fluid, or possibly a gelatinous substance. The cytoplasm of the 

 distended cell is so stretched that it appears as the thinnest possi- 

 ble layer surrounding the vacuole. The very flat nucleus occasions a 

 bulge in the contour of one side of the cell (Fig. 265). The outer cell- 

 membrane, while very thin, is probably of semirigid consistency. Seen 

 under the microscope, this tissue looks like a mass of soap bubbles 

 crowded closely together, the cytoplasm and cell-membrane of each 

 cell being the wall of a bubble. 



The vacuolated notochord tissue is enclosed by sheaths which differ 

 in number and nature in various animals. There is commonly an inner 

 elastic sheath (Fig. 265, ei) composed of material secreted by an outer 

 epithelioid layer of the notochord tissue, and a thick outer sheath of 

 dense fibrous connective tissue. Mechanically, the notochord resembles 



