330 Basic Structure of Vertebrates 



some relation to the clotting of blood, as indicated by the fact that the 

 filaments of fibrin tend to radiate from blood-platelets. 



Lymph resembles blood but lacks erythrocytes and is therefore 

 colorless. The fluids occupying the coelomic spaces and the cavities of 

 brain and spinal cord, the aqueous humor of the eye, and the amnionic 

 fluid are all of the general nature of lymph but contain relatively few 

 cells and differ from one another in details of chemical constitution. 



Histologic Specificity 



In general, histologic differences are less conspicuous than the corre- 

 sponding anatomic differences. Nonstriated muscle-fibers appear much 

 the same whether they are in the wall of a stomach or of a lung. 

 Nevertheless, cells and tissues usually exhibit characteristics which 

 mark them as belonging to a particular organ or animal. The nerve- 

 cells of a spinal ganglion differ from the motor nerve-cells in the spinal 

 cord of the same animal. Striated muscle of the vertebrate heart differs 

 from that of the body-wall. Vertebrate cardiac muscle differs from that 

 of a lobster. Epidermal tissue of a fish differs from that of a reptile. 



It follows, therefore, that the individual tissue-cell may, in its 

 visible structure, exhibit characteristics reflecting as many as four 

 grades of organization. First, there are those cell organs, such as nu- 

 cleus and chromatin bodies, which represent the fundamental organi- 

 zation of protoplasm as cells. Second, there are those intracellular 

 structures such as myofibrils or neurofibrils which mark the cell as 

 belonging to a particular tissue — muscular or nervous. Third, there 

 may be features which identify the cell as belonging to tissue of a cer- 

 tain organ — for example, the intercalated disks in the heart muscle of 

 vertebrates. Finally, the individual tissue element may have peculiari- 

 ties which are specific for animals of a certain group; for example, the 

 striated muscle-fiber of an insect differs in details of structure from 

 that of a vertebrate. From bottom to top of this series of levels of 

 protoplasmic organization — cell, tissue, organ, individual, species, 

 etc. — there must be corresponding chemical specificity. To a con- 

 siderable extent, it has been demonstrated. 



Fig. 271. (See facing page.) Cells from smear preparation of normal human 

 blood; Wright's stain. (Center) Adult red blood-corpuscles, blood-platelets, and a 

 polymorphonuclear neutrophil. (Top, left) Two polymorphonuclear basophils and 

 two polymorphonuclear eosinophils. (Top, right) Three large and four small 

 lymphocytes. (Bottom, left) Polymorphonuclear neutrophils; two of these cells, 

 the uppermost and lowermost of the group, are young, with merely crooked 

 nuclei; the mature cells have multilobed nuclei. (Bottom, right) Six monocytes; in 

 the younger cells the nuclei tend to be rounded; in the adult cells they are horse- 

 shoe-shaped, indented, or lobed. (Courtesy, Bremer- Weatherford: "Text-Book of 

 Histology," Philadelphia, The Blakiston Company.) 



