THE BLOOD CELL 97 



or the polynuclear leucocytes, with neutral-staining granules. These 

 comprise from 65 to 75 per cent of the total count. 



Another type of white cell is known as the transitional cell, because 

 it was supposed to represent an intermediate form between the mono- 

 and polynuclear cells. Probably such transitions do not occur, and the 

 transitional leucocyte is related to the mononuclear cells. 



The polynuclear cells originate in the bone marrow, and for this 

 reason have been termed myeloid cells. They develop from cells in 

 the bone marrow termed myeloblasts, which are nongranular and con- 

 tain a large nucleus. In the course of development the characteristic 

 granules appear, and the nucleus remains round and later becomes 

 lobulated. These intermediate forms are called myelocytes. The mono- 

 nuclear cells originate in the lymphatic tissues of the body. 



The leucocytes possess the ability to make ameboid movement and 

 to ingest foreign particles which may be presented to them. On ac- 

 count of this latter ability they are commonly called phagocytes. In 

 the process of inflammation the leucocytes assemble at the spot which 

 is the seat of the injury or infection, and remove the foreign organism 

 or necrotic tissue by ingesting and digesting it. 



It is not definitely known whether or not the lymphocytes func- 

 tion as phagocytes. Other functions besides those as phagocytes have 

 been ascribed to the white cells, but they are not universally ac- 

 cepted. The number of leucocytes in the blood is subject to con- 

 siderable variation. They normally number between 6,000 and 8,000 

 per c.mm. At the height of digestion and after strenuous exercise 

 there is usually a small increase, and under pathological conditions, 

 especially in infectious diseases, this becomes quite marked. Some 

 infections increase the polymorphonuclear cells, while others add to 

 the lymphocytes. The factors governing the type of increase are not 

 fully known, nor are the functions of the various forms differentiated. 



The Blood Platelets 



These are small oval particles about 3 //, in diameter, which are found 

 in large numbers (250,000 to the c.mm.) in the blood. They are sup- 

 posed to be formed from particles of protoplasm which are pinched 

 off from the large blood cells in the bone marrow. Their biological 

 and chemical properties are not understood. They probably play a 

 very important role in the coagulation of the blood (see page 103). 



