688 HUMAN ANATOMY. 



to maintain the normal quota are made. The production of the new red cells within 

 the marrow proceeds from slightly colored elements, \heerythroblasts, that by division 

 give rise to normoblasts and nucleated erythrocytes, which latter, upon the distribu- 

 tion of hemoglobin and the 

 Fig. 6si. disappearance of their nuclei, 



are transformed into the usual 

 red cells, and as such enter 

 the circulation. 



The disappearance of the 

 nucleus of the normoblasts has 

 long been a subject of discus- 

 sion and speculation. Accord- 

 ing to the older view — still, 

 however, accepted by many — 

 the nucleus is extruded from 

 the erythrocyte and under- 

 goes disintegration, thus, in 

 the opinion of some, supply- 

 ing the source of the blood 

 plaques. According to the 

 more recent views, held bv 



Developing blood-vessels in embryonal subcutaneous tissue ; NcUmaUU, KblHker, PappCU- 



a, larger capillary; b. young capillaries; c, solid protoplasmic Vipim Tsrflpi Fhner Tnd others 



outgrowths forming new vessels. X 300. iieiiii, ibidci, jz^uiici , aiiuuLiicis, 



the disappearance of the nu- 

 cleus is due to its solution and absorption within the erythrocyte. Under normal 

 conditions the immature nucleated red cells do not occur in the circulation. After 

 severe hemorrhage or in other conditions requiring unusual activity of the blood- 

 forming processes, they may be present in large numbers until the normal quota of 

 erythrocytes has been once more established. In view of the constant presence of 

 normoblasts and nucleated erythrocytes within the splenic pulp, the spleen has been 

 regarded as a possible, althougfh under usual conditions limited, source of the red 

 blood-cells. When, however, the necessity for rapidly augmenting the number of 

 red cells arises, the spleen may assume the role of an active blood-producing tissue. 

 Since such cells are found also in the thymus, this body probably may be included 

 among the blood-forming organs of early life. There is no satisfactory evidence that 

 the erythrocytes are derived from the colorless cells or from the blood plaques. 



Development of the Colorless Cells. — Immediately succeeding the appear- 

 ance of the primary red cells, the latter are the chief elements within the circulation. 

 In the early weeks, however, colorless cells appear and henceforth are the com- 

 panions of the erythrocytes. As already noted, the white 

 cells are elements that primarily belong to the lymphatic Fig. 652. 



system, from which they are poured into the blood chan- 

 nels. Genetically, the red and white cells are unrelated. 



Concerning the origin of the first colorless cells un- a^ es^^ 



certainty exists, although it is generally assumed that ^ © 



they arise from mesoblastic cells and, therefore, to that © ^^^ 



extent, share with the erythrocytes a common source. @ © ^^^ - 



According to Maximow,i the progenitors of the white /^^.^  © ®' 



cells are lymphoblasts, derived from the primitive blood- © ^-r''^ (Sa 



cells, tho. h€77ioblasts ; the latter are, therefore, the source \%)j "--^V 



of both the red and white cells, including the various . --'' 



forms of the colorless corpuscles. The conclusion of 

 Beard, 2 that the first lymphocytes to appear within the , Nucleated embryonal eryihro- 



' . 1 • 1 1 • c cytes; two dnidmg cells exhibit 



embryo owe their production to the metamorphosis of mitotic figures, x 600. 



the entoblastic epithelium of the primary thymus, and 



that the subsequent migration of the lymphocytes so derived establishes foci from 



which are developed the various masses of lymphoid tissue occurring throughout the 



1 Archiv f. mikros. Anatom., Bd. Ixxiii., 1909. 

 •Anatom. Anzeiger, Bd. xviii., 1900. 



