196 THE BLOOD 



During the early stages of embryonic existence, the precursors 

 of the red corpuscles, generally known as erythroblasts, are large 

 and nucleated, while the non-nucleated cells which are so char- 

 acteristic of the adult animal, appear at a much later time. In the 

 human fetus, for example, all the cells are nucleated at the end of the 

 fourth week, while at the end of the third month only about one- 

 fourth of their total number is still in possession of a nucleus. The 

 corpuscles of the latter type become fewer and fewer in number as 

 gestation advances until at birth practically all the circulating ery- 

 throcytes are without a nucleus. Only those which are still retained 

 in the corpuscle-forming, or hematopoietic tissues, remain nucleated. 

 Naturally, the loss of the nucleus which occurs either by disintegration 

 or extrusion, implies that they are now fully developed and also, that 

 they no longer multiply by simple division. 



The formation of the red corpuscles does not cease at the end of 

 intrauterine existence, but is continued throughout the life of the 

 animal; and furthermore, as their number does not increase, their 

 formation must be counterbalanced by an adequate destruction. 

 That this is true may be inferred from many experiments. Thus, if 

 a loss of red corpuscles is effected by bleeding, the fluid parts of the 

 blood are quickly replaced by transferring a certain quantity of the 

 tissue-lymph into the vascular system. Consequently, the blood is 

 relatively poor in corpuscles directly after the hemorrhage, but ac- 

 quires them in greater numbers later on as new ones are sent in by the 

 hematopoietic tissues. An interval of a few days generally suffices 

 to establish the normal corpuscle count, but naturally, much depends 

 upon the quantity of blood lost and the activity of the corpuscle- 

 forming tissues. A second fact that should be mentioned at this time 

 is the constant outgo of pigmentous material in the feces and urine, 

 in the form of urochrome, urobilin and stereobilin. It has been shown 

 that these substances originate in the liver and that their production 

 is closely dependent upon the amount of hemoglobin available for 

 this purpose. By inference, therefore, it may be concluded that a 

 supply of this coloring material must be constantly at hand; i.e., 

 it must be brought to this organ by the red cells in undiminishing 

 quantities. 



During extrauterine life the erythrocytes are formed in the red 

 marrow of the bones. Marrow of this color is found in the flat and 

 short bones of the head and trunk and in the long bones of the ex- 

 tremities. The latter, however, contain it solely in their ends. It is 

 also to be noted that the yellow marrow in the other regions of these 

 bones may assume the characteristics of red marrow at any time when 

 a very active regeneration of the red cells is called for. The fatty 

 marrow in the diaphyses then becomes filled with a red pasty mass 

 consisting chiefly of red cells and their precursors. This conversion 

 may readily be induced in animals by bleeding. A similar change has 

 been observed in hibernating animals. Red marrow is formed very 



