328 



Basic Structure of Vertebrates 



nucleus lies in the peripheral layer of cytoplasm. The irregular poly- 

 hedral form of the cells is doubtless the result of their mutual pressures. 

 As seen in microscopic sections, it looks very much like tissue of a 

 notochord. 



The circulatory function of blood requires that it be fluid, but 

 various special services are rendered by cells suspended in the fluid, 

 some of them passively carried by it, others capable of independent 

 motion somewhat like that exhibited by an ameba. 



Fig. 270. Coagulated blood. Biconcave red cor- 

 puscles arranged in "rouleaux"; filaments of fibrin 

 radiating from minute blood-platelets. (After Da 

 Costa. Courtesy, Bremer: "Text-Book of Histology," 

 Philadelphia, The Blakiston Company.) 



The fluid part of blood, the plasma, is water containing all the 

 other substances which enter into the constitution of protoplasm to- 

 gether with various hormones and the waste products of metabolism. 

 Derived from digested food are various dissolved proteins and sugar, 

 and minute fat droplets are suspended in the watery plasma. In its 

 inorganic chemical constitution, the plasma resembles sea water. 



In the coagulation of blood, on exposure to air or under some other 

 circumstances, a nitrogenous substance, fibrinogen, carried by the 

 plasma in solution, becomes transformed into fine solid filaments of 

 fibrin (Fig. 270). The uncoagulated portion of the plasma is called 



