Picturesquely speaking, the blood may be regarded as an organ in which, in a liquid medium called the plasma, various cellular elements, all necessary to life, are brought into intimate contact with every part of the living body. All vertebrates have red blood, with the exception of the amphioxus, which has white blood. This cordate animal is possibly the connecting link between the invertebrates and the vertebrates. All invertebrates that have a circulating medium have white blood, although a few have blood cells showing a trace of hemoglobin. The first blood in the human embryo is white, and there is evidence of a primitive mechanism for circulating white blood that has become a part of the vascular system.
Fairly accurate knowledge of the structure of the cellular elements of the blood has been gained through the microscope, but the constituents of the plasma lie in the colloid and molecular fields
MAYO WJ. THE INTERRELATION OF LESIONS OF BONE MARROW, LIVER AND SPLEEN. JAMA. 1926;87(20):1609–1613. doi:10.1001/jama.1926.02680200009004
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