100 



NORMAL HISTOLOGY. 



FIG. 136. Embryonal blood ; the 

 dividing erythroblasts are producing 

 nucleated erythrocytes. X 600. 



and become the ordinary erythrocytes or red blood-cells. This loss of the 

 nuclei, effected probably by fragmentation and absorption and not by extru- 

 sion, begins in the human embryo about the second month, but is not 

 completed until towards the close of foetal life. Even at, or for a few 

 weeks after birth, occasional nucleated erythrocytes may be encountered in 



the circulation. During the last months of 

 foetal life, the erythroblasts retire more and 

 more within the red bone-marrow, which, after 

 birth, becomes the chief, if indeed under normal 

 conditions not the exclusive, seat of the pro- 

 duction of new red-cells during life. Hence 

 the significance and frequent classification of this 

 tissue (page 40) as a blood-forming organ. 



The Colorless Cells. Our knowledge 

 concerning the origin of the earliest white 

 blood-cells is incomplete and the views concern- 

 ing the genetic relations of the lymphocytes and 

 the leucocytes are far from accord. Although 

 the exact sequence is uncertain, it is generally 

 assumed that these cells arise from mesodermic 

 elements, and to that extent, at least, share 



a common origin with the erythrocytes. Further, that the white cells are 

 formed outside the blood-vascular system, which they subsequently enter. 

 The assumption, that the first lymphocytes are formed in loco within the 

 early thymus body, by the metamorphosis of the entodermic epithelium, and 

 that the subsequent migration of lymphocytes so derived establishes foci 

 from which are developed 

 the masses of lymphoid tis- 

 sue throughout the body, 

 has now been abandoned. 

 The progenitors of the 

 leucocyte-group of color- 

 less cells seem to be large 

 elements, the myeloblasts, 

 which appear in the devel- 

 oping liver and the early 

 bone-marrow and possess 

 abundant cytoplasm devoid 

 of granules. These prima- 

 ry elements give rise to the 

 myelocytes, which exhibit a 

 granular cytoplasm and are 

 found chiefly in the bone- 

 marrow and, to a limited 

 extent, the spleen. From 

 the myelocytes descend the 



various forms of the leucocytes and, probably, the huge mononuclear 

 marrow-cells, the megakaryocytes. The lymphocytes, on the other hand, 

 are the especial derivatives of the lymphoid tissues, within the so-called 

 germ-centres in which they arise by mitotic division. The lymph-nodes, 

 the spleen and the red bone-marrow are, therefore, the most important seats 

 of the production of the colorless blood-cells. From the standpoint of 

 early development, the sharp distinction between the lymphocytes and the 



FIG. 137. Section of embryonal bone-marrow, showing nucleated 

 erythrocytes, leucocytes and megakaryocyte. X 625. 



