DEGA Y OF RED BLOOD-CORPUSCLES. 95 



The origin of red blood-corpuscles is not fully determined. 

 The earliest red corpuscles of embryonic life originate along 

 with the bloodvessels from mesoblastic cells (" angioblasts ") 

 which form a protoplasmic network, in the substance of 

 which, and especially at the intersections, nuclei proliferate 

 and accumulate. These masses of multinucleated protoplasm 

 break up, becoming converted into cavities filled with sepa- 

 rated cells; the peripheral protoplasm and nuclei form an 

 endothelial lining to the cavity, which is now an incipient 

 bloodvessel, while the central mass of nucleated protoplasm 

 separates into amoeboid nucleated cells, which then acquire 

 haemoglobin and become nucleated red blood-corpuscles. 

 From these fi blood-islands " the process of vessel and cor- 

 puscle formation proceeds along the protoplasmic strands. 



Thus the earliest embryonic red corpuscles are nucleated, 

 and capable of self-reproduction by indirect division ; these, 

 however, soon disappear, and are replaced during the remain- 

 der of life by non-nucleated corpuscles. 



The most probable source of red corpuscles in the adult 

 seems to be the bone-marrow. As described above, bone- 

 marrow contains an ample network of expanded vascu- 

 lar sinuses, and one class of the cells of the marrow 

 consists of the erythroblasts, small nucleated haBmoglobin- 

 containing cells capable of amoeboid movement. The red 

 corpuscles are probably produced from these cells, but in 

 what manner is not known ; the erythroblasts themselves may 

 perhaps be descendants of the nucleated red corpuscles of the 

 embryo, continued in existence by repeated division. 



Other theories of the origin of red corpuscles have been 

 presented, as that they are produced from leukocytes, or blood- 

 plates, or in the spleen, lymphatic glands, or thy m us, but 

 these suppositions have not been well substantiated. There 

 is a constant destruction of erythrocytes, and a means some- 

 where in the body by which they can be actively renewed. 

 They cannot reproduce themselves by division ; and the only 

 known seat for their formation supported by any great 

 probability is the bone-marrow. 



Decay of red blood-corpuscles : Erythrocytes, like all other 

 living cells, pass through a life-cycle of development, matur- 

 ity, decay, and death. The red corpuscles seen in normal 



