DEVELOPMENT OF THE BLOOD -CORPUSCLES 



11 



In later embryonic life, and after birth, nucleated coloured cor- 

 puscles are no longer present in mammalian blood, but are replaced 

 by the usual discoid corpuscles. These are formed within certain cells 

 of the connective tissue, a portion of the protoplasm of the cell becom- 

 ing coloured by haemoglobin, and separated into globular particles 

 (fig. 9, h, h', h"), which are gradually moulded into disk-shaped red 

 corpuscles. In the meantime the cells become hollowed out, and join 

 with similar neighbouring cells to form blood-vessels (fig. 10, a, b, c}. 

 The process is therefore the same as before, except that the cell-nuclei 

 do not participate in it. 



FIG. 10. FURTHER DEVELOPMENT OF BLOOD-CORPUSCLES IN CONNECTIVE-TISSUE CELLS, 



AND TRANSFORMATION OF THE LATTER INTO CAPILLARY BLOOD-VESSELS. 



a, an elongated cell with a cavity in its protoplasm occupied by fluid and by blood-corpus- 

 cles mostly globular; &, a hollow cell the nucleus of which has multiplied. 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 a ' haemapoietic ' cell, which in this in- 

 stance 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. 



Although no nucleated coloured corpuscles are to be seen in the 

 blood in post-embryonic life, they continue to be found in the marrow 

 of the bones (see Lesson XIII.), and it is thought probable that the 

 red disks may be formed in some way from these. Others have sup- 

 posed that the red disks are derived from the white corpuscles of the 

 blood and lymph, and others again that they are developed from the 

 blood-tablets ; but the evidence in favour of these views is insufficient. 



The white blood-corpuscles and lymph-corpuscles occur originally 

 as free unaltered embryonic cells, which have found their way into the 

 vessels from the circumjacent tissues. Later they become formed in 

 lymphatic glands and other organs composed of lymphoid tissue, and 

 pass from these directly into the lymphatics and so into the blood. 



