HISTOLOGY 153 



This tissue forms the supporting framework of such organs as lymph 

 glands and the spleen. In form and arrangement its cells resemble 

 embryonic mesenchyme cells. The spaces within the network are occu- 

 pied by a fluid substance in which are numerous lymphocytes and other 

 cells. 



Tendons and ligaments are connective-tissue structures highly adapted 

 to resisting tensile strain. Tendons (Fig. 113C) consist of coarse collage- 

 nous fibers lying parallel to one another in compact bundles. Tendons are 

 inelastic. The elasticity necessary to a ligament is due partly to a certain 

 degree of elasticity in the individual fiber and partly to the fact that the 

 fibers are wavy in contour when not under strain and are more or less 

 interwoven. 



Chromatophores, pigment cells (Fig. 1135), may occur in connective 

 tissue, especially in the dermal layer of the skin. The specific pigment 

 appears as granules lying in the cytoplasm. Black pigment (melanin) is 

 most common and cells containing it are called melanophores. Chroma- 

 tophores 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. 



Skeletal Tissues 



Notochord. Among skeletal structures the notochord is histologically 

 unique in that its tissue is for the most part neither hard nor even solid; 

 it is fluid. It is exceptional also in its embryonic origin. It is not a 

 product of mesenchyme but arises simultaneously with the mesoderm and 

 in much the same way although independently of it (see page 86). 



The essential notochord material consists of cells each of which contains 

 a relatively enormous vacuole occupied by a substance of fluid or possibly 

 gelatinous consistency. The cytoplasm of the distended cell is so 

 stretched that it appears as the thinnest possible layer surrounding the 

 vacuole. The very flat nucleus occasions a bulge in the contour of one 

 side of the cell (Fig. 116). The outer cell-membrane, while very thin, is 

 probably of semi-rigid consistency. Seen under the microscope, this 

 tissue looks hke a mass of soap bubbles crowded closely together, the cyto- 

 plasm and ceU-membrane of each cefl being the waU 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. 116, ei) composed of material secreted by an outer 

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

 fibrous connective tissue. 



The capacity of the notochord as a whole to resist deforming strains is 

 doubtless due in part to the cell walls of the vacuolated tissue, these walls 



