170 HANDBOOK OF PHYSIOLOGY. 



very extensively developed, long after the appearance of red cells; the 

 exact time of their appearance, however, has not yet been fully deter- 

 mined. It is now quite generally believed that in the foetus both red 

 and white cells are derived from a common origin, and that they become 

 differentiated in the course of development. The earliest known pro- 

 genitors of the leucocytes are the primary wandering cells of me.sodermal 

 origin, which are found chiefly in the connective tissues, thus lying out- 

 side of the vessels. Gathering in groups, partly at the sites of the 

 future lymph nodes, but chiefly in the embryonal liver, these wandering 

 cells pass through several generations of mitotic division, and thus 

 gradually assume the type of leucocytes. According to a few observers 

 the leucocytes are also formed in the circulating blood and lymph by 

 amitotic, less frequently by mitotic, division. Later on in fcetal life 

 the function of forming leucocytes is gradually transferred from the 

 liver to the lymphoid and adenoid tissues, i.e., the lymph nodes, spleen, 

 marrow, and thy m us. 



In adult life, under normal conditions, the leucocytes are only 

 formed in the lymphoid tissues, including the lymph nodes, spleen and 

 marrow. The process is also one of mitotic division (a few authorities 

 claim that it is amitotic), and the resulting cells pass into the circulation 

 by way of the thoracic duct. 



Uses of the Blood. 



1. To be a medium for the reception and storing of matter, e.g., 

 oxygen and digested food material, from the outer world, and for its 

 conveyance to all parts of the body. 



2. To be a source whence the various tissues of the body may take 

 the materials necessary for their nutrition and maintenance; and whence 

 the secreting organs may take the constituents of their various secre- 

 tions. 



3. To be a medium for the absorption of refuse matters from all the 

 tissues, and for their conveyance to those organs whose function it is tt 

 separate them and cast them out of the body. 



4. To warm and moisten all parts of the body. 



