Development of Erythrocytes 



261 



the introduction of any complicating factors. When feeding 

 is resumed and observations are made at varying intervals there- 

 after, it is possible to secure a series of marrows of various de- 

 grees of complexity, as normal cellularity is approached. An 

 analysis of each stage, from the simple depleted to the normally 

 cellular state, enables a clarity of interpretation and understand- 

 ing of the normal process of blood-cell formation and develop- 

 ment hitherto unknown. The phenomena revealed by this 

 series have proved highly suggestive. 



Forty-eight hours after the resumption of feeding in an ani- 

 mal with a previously induced hypoplasia, a most remarkable 

 change in the appearance of the marrow is to be seen. The ex- 

 cessive deposit of fat has very largely disappeared. This is 

 evidenced by a marked stellate appearance of the shrinking fat 

 cells, with the fat therein divided into many various sized glo- 

 bules, in contrast to the hypoplastic state where it was deposited 

 as one homogeneous mass in the large fat cells. In this stage 

 the fat cells simulate clasmatocytes in appearance, and are most 

 numerous along the outsides of the blood vessels. There is in 

 addition to this a most striking proliferation of endothelium. 

 Because of the prominence of the strands of endothelial cells, 

 their distribution is very readily appreciated. There remains no 

 doubt in this stage that the inter-sinusoidal channels which in 

 injected hypoplastic marrow are seen surrounding the fat cells, 

 are true endothelial capillaries, for the proliferated endothelium 

 follows precisely and accurately their pericellular, intersinusoidal 

 outline. The picture described is not that seen in a few isolated 

 areas, but it is a transformation in which the entire marrow sub- 

 stance participates. Here and there are to be seen a few young 

 red blood cells, all intravascular, and in contact with the swollen 

 endothelial cells of a collapsed capillary. In the extravascular 

 parenchymal spaces there is to be seen an occasional so-called 

 "reticular cell' in the process of mitotic division. The "reticular 

 cells" of the hypoplastic state are large irregular pentagonal or 

 hexagonal cells with faintly staining eosinophilic cytoplasm, and 

 round vesicular nuclei; simulating in appearance primitive 

 mesenchyme. 



In the marrows analysed after seventy-eight hours of stimu- 

 lation most of the fat deposit has entirely disappeared, and only 

 a few vacuolated cells remain. There is still more extensive 

 proliferation of the capillary endothelium than at forty-eight 



