460 THE BLOOD [CH. XXIX. 



mostly spherical, and with granular contents, and a well-marked 

 nucleus. 



The corpuscles then strongly resemble the colourless corpuscles 

 of the fully developed blood, but are coloured. They are capable of 

 amoeboid movement and multiply by division. 



These coloured nucleated cells begin very early in foetal life to 

 be mingled with coloured T^w-nucleated corpuscles resembling those 

 of the adult, and at about the fourth or fifth month of embryonic 

 existence are completely replaced by them. 



These coloured discs are partly formed in connective-tissue 

 cells in a way similar to that just described, only without the 

 participation of the nuclei in the process, although there is very 

 little doubt that haemoglobin originates from the haematogen (iron- 

 containing nuclein) of the nuclei in all cases. The foetal liver, 

 spleen, and thymus are also believed to be seats of formation of 

 the red discs. 



Without doubt, the red corpuscles have, like all other parts 

 of the organism, a tolerably definite term of existence, and in a like 

 manner die and waste away when the portion of work allotted to 





FIG. 326. Coloured nucleated corpuscles, from the red marrow of the guinea-pig. 

 (E. A. Schiifer.) 



them has been performed. Neither the length of their life, however, 

 nor the fashion of their decay, has been yet wholly made out. A 

 certain number of the coloured corpuscles undergo disintegration in 

 the liver and spleen ; corpuscles in various degrees of degeneration 

 have been observed in both these organs. 



This being so, it is necessary that the red corpuscles should be 

 constantly replenished throughout life. But after the festal stage 

 is passed, they originate, not from connective tissues in general, but 

 in one special form of connective tissue, namely, the red marrow 

 of bones. It is possible that in some animals the spleen, which 

 contains cells very similar to those of the marrow, may participate 

 in their formation. In the red marrow, they arise from immature 

 nucleated cells (normoUasts or erythroNasts, fig. 326) ; the nucleus is 

 not discharged, but is absorbed within the cell, and this is the explana- 

 tion that some observers give of the biconcave form of the red disc. 

 Sometimes immature nucleated red cells may make their way from 

 the marrow into the circulation ; and the free nuclei of these cells 

 are sometimes found in the blood ; they never, when once they have 

 entered the blood, develop into discs, and are filtered out of the 

 blood by the spleen. 



