14 



THE ESSENTIALS OF HISTOLOGY. 



become hollowed out, and join with similar neighbouring cells to form 

 blood-vessels (fig. 13, a, b, c). The process is therefore the same as 

 before, except that the cell-nuclei do not participate in it. 



Although no nucleated coloured corpuscles are to be seen in the 

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

 of the bones (see Lesson XIII. ), and in some animals they have also 

 been found in the spleen. It is thought probable that the red disks 

 may be formed from these by the nucleus disappearing and the coloured 

 protoplasm becoming moulded into a discoid shape. Others have sup- 



FIG. 12. BLOOD-CORPUSCLES DEVELOPING WITHIN CONNECTIVE-TISSUE CELLS. 



a, a cell containing diffused haemoglobin ; 6, a cell filled with coloured globules ; c, a cell con- 

 taining coloured globules in the protoplasm, within which also are numerous vacuoles. 



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 



FIG. 13. FURTHER DEVELOPMENT OF 

 BLOOD-CORPUSCLES IN CONNECTIVE- 

 TISSUE CELLS, AND TRANSFORMA- 

 TION OF THE LATTER INTO CAPIL- 

 LARY BLOOD-VESSELS. 



., an elongated cell with a cavity in its 

 protoplasm occupied by fluid and by 

 blood-corpuscles mostly globular ; 6, 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 instance contains only 

 one corpuscle, with the prolongation 

 (bl) of a previously existing vessel, a, 

 and c, from the new-born rat ; &, from 

 a foatal sheep. 



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

 vessels from the circumjacent mesoblast. 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. 



