THE BLOOD AND LYMPH. 123 



the endothelial celLs are active in bringing about these differences. 

 Their character is not such as would be expected of cells carrying 

 on active processes. 



The red corpuscles are soft, elastic discs, with a concave impres- 

 sion in both surfaces (Fig. 105). They are slightly colored by a 

 solution of haemoglobin, and are so abundant that their presence 

 gives the blood an intense red color ; but when viewed singly under 

 the microscope each corpuscle has but a moderately pronounced red- 

 dish-yellow tinge. The haemoglobin solution is either intimately 

 associated with the substance composing the body of the corpuscles, 

 called the " stroma," or it occupies the centre of the corpuscle and 

 is surrounded by a pellicle of stroma. 



Under normal conditions the red corpuscles, in man and most 

 of the mammalia, are not cells, for they possess no nuclei, nor are 

 they capable of spontaneous movement or multiplication. They 

 are, rather, cell-products, being formed either within the cytoplasm 

 of cells of mesoblastic origin, or by the division of cells derived 

 from the mesoblast, and called erythroblasts, the descendants of 

 which become converted into red corpuscles through an atrophy and 

 disappearance (probably expulsion) of the nuclei and a transforma- 

 tion of the cytoplasm into the stroma, which take place after the 

 elaboration of the haemoglobin within the cell. The former, or 

 intracellular, mode of production occurs in the embyro, even before 

 the complete development of the bloodvessels ; the latter mode of 

 production seems to be the only one occurring in the adult, the chief 

 location of the erythroblasts appearing to be in the red marrow of the 

 bones, where they are situated either in the tissues of the marrow 

 itself, whence their descendants, while still cellular, pass into the 

 vessels, or in the large venous channels of the marrow, where the 

 blood-current is sluggish and the erythroblasts remain close to the 

 vascular walls. In some anaemic conditions the erythroblasts ap- 

 pear in the circulating blood, where they may be distinguished from 

 the normal red corpuscles by the presence of their nuclei and, fre- 

 quently, also by a difference in size (see Fig. 103, c). 



In the reptilia and birds the red corpuscles are normally nu- 

 cleated ; but, though morphologically resembling cells, they are 

 incapable of multiplication or spontaneous movement, and have 

 undergone such modifications that they are not cells in a physiolog- 

 ical sense. 



The functional value of the red corpuscles is dependent upon the 



