Comparative Hematology and the Functions of the Leucocytes

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Abstract
The vertebrates and most of the invertebrates have free cells of mesenchymatous origin. In some groups these cells are more or less confined to vascular and lymphatic vessels; in others they wander through the tissue spaces. In all groups that have been satisfactorily investigated certain fundamental types of cells are found: Relatively undifferentiated cells that have the capacity to differentiate into other types of blood cells or wandering cells, and, in some groups and possibly in all, into any type of cell in the body including the genital cells. Phagocytic cells that serve the function of consuming foreign materials and dead or damaged tissue of the animal itself. A greater or smaller number of various types of more specialized granular and vacuolated cells that contain nutritive materials and give evidence of enzyme activity. In many low forms these constitute an important part of the food handling and digesting mechanism. In other forms, including the vertebrates, their original function has been taken over largely by the digestive glands of the gut tract. Of limited distribution, present in animals of some groups but not in others, are specialized cells of a variety of types serving other special functions characteristic for the animals in which they are found. To be included here are such cells as the cells with respiratory pigments found in the vertebrates and some invertebrates with high oxygen requirements; the carotene bearing cells and the excretory cells of some ascidians; and the flagellate cells of some echinoderms which, in the absence of a heart, keep the fluid in circulation.

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