942 PHYSIOLOGY 



inside the dilated capillaries and intermediate stages between the 

 erythroblasts, and the fully formed erythrocytes are almost entirely 

 wanting. In the frog this process of blood- corpuscle formation occurs 

 only at one period of the year, namely, in the early summer, and it is 

 only at this time that the bones are found to contain red marrow. In 

 mammals the process is very similar. In the red marrow are a number 

 of nucleated cells containing haemoglobin, which are thought by 

 Lowit to be themselves derived from colourless nucleated cells. In 

 the confused medley of colourless cells which exists in the bone- 

 marrow and are precursors of all the varied corpuscles found in the 



FIG. 362. Section of red marrow of pigeon. 



Ic, eosinophile leucocytes ; eg, fat cells ; e, nucleus of endothelial 

 cell of blood-vessel ; ca, blood-capillary ; er, erythroblasts lying 

 within vascular endothelium ; glr, fully formed red corpuscles. 



blood, it is difficult to be certain of the identity of the colourless 

 erythroblasts and to distinguish them from the smaller colourless 

 cells engaged in bone formation or in the production of leucocytes. 

 The haemoglobin-containing cells are often to be seen in process of 

 division, and the nucleated daughter-cells appear to undergo a process 

 of nucleolysis, the nucleus being extruded or dissolved. When blood 

 formation is quickened as the result of previous destruction or loss, 

 some of these immature nucleated blood- discs may make their way 

 into the circulation and be found in the blood, where they are spoken 

 of as normoblasts. 



How long a corpuscle continues to exist in the circulating blood is 

 not known. The experiments made to determine the length of time 

 during which foreign corpuscles such as those of birds can be recognised 

 after injection into the circulation of a mammal are evidently beside 

 the mark, since these foreign cells will be destroyed by the serum and 



