ORIGEN OF THE RED BLOOD-DISKS. 



219 



neighbouring blood-vessels or of similar cells. At the same time vacuoles form 

 within them (fig. 255, h'), and becoming enlarged coalesce to form a cavity filled with 

 fluid, in which the reddish globules, which are now becoming disk-shaped, float 

 (fig. 256). Finally the cavity extends through the cell -processes into those of neigh- 

 bouring cells, and a vascular network is produced, and this becomes eventually 

 united with pre-existing blood-vessels, so that the blood-corpuscles which have 

 been formed within the cells in the manner described, get into the general circu- 

 lation (see Development of Blood-vessels). 



This " intracellular " mode of development of red blood-corpuscles ceases in most 

 animals before birth, although in those which, like the rat, are born very immature, 

 it may be continued for a few days after birth. Subsequently, although new vessels 

 are formed in the same way, blood-corpuscles are not produced within them, and it 



Fig. .256. FURTHER DEVELOPMENT OF BLOOD-CORPUSCLES 



WITHIN CONNECTIVE TISSUE CELLS, AND TRANSFORMATION 

 OF THE LATTER INTO CAPILLARY BLOOD-VESSELS (E. A. S.) 



a, an elongated cell with a cavity in its protoplasm 

 occupied by fluid and by blood-corpuscles which are still 

 globular ; b, a hollow cell the nucleus of which has multi- 

 plied. The new nuclei are arranged around the wall of the 

 cavity, the corpuscles in which have now become discoid ; 

 c, shows the mode of union of an angioblast, which in this 

 instance contains only one corpuscle, with the prolongation 

 (bl) of a previously existing vessel, a, and c, from the 

 new-born rat ; b, from a foetal sheep. 



becomes necessary to seek for some other source 

 of origin of the red blood-discs, both during the 

 remainder of the period of growth, and also 

 during adult life/for it is certain that the blood- 

 corpuscles are not exempted from the continual 

 expenditure and fresh supply which affect all the 

 other tissues of the body. 



2. In the marrow of bones. In the red marrow which fills the internal 

 cavities of many bones, and particularly the ribs, corpuscles have been observed 

 which appear to justify the inference that red blood-corpuscles are here becoming 

 developed. These corpuscles, which have been termed erythrollasts, were long ago 

 described by Neumann, and by Bizzozero, and have since been noticed also by many 

 other observers, by most of whom they are stated to be formed from the leucocytic 

 marrow-cells. The accounts are, however, somewhat different ; for, according 

 to some, the nucleus of the marrow-cell becomes coloured, and with a small amount 

 of protoplasm persists as the red disc, while others describe the protoplasm as 

 becoming transformed into the red corpuscle whilst the nucleus disappears. 



According to the account given by Bizzozero, the erythroblasts are not 

 developed from the leucocytic marrow cells, nor from the white corpuscles of the 

 blood, but are corpuscles sui generis, which multiply by karykinesis, and become 

 gradually transformed, in the mammalia with disappearance of the nucleus, into the 



Fig. 257. COLOURED NUCLEATED CELLS FROM THE RED MARROW OF THE GUINEA-PIO (E. A. S.) 



red blood-disks. My own observations are entirely in accordance with these state- 

 ments. The coloured cells that I have noticed have almost always been distinctly 

 smaller than the ordinary marrow-cells, often of irregular forms, and sometimes 

 vox,, i. Q 



