Development of Erythrocytes 



263 



(2) that the growing clumps of red cells cause erosion of the 

 endothelial lining by pressure, and hence obtain entrance to 

 the vascular bed. Drinker, Drinker and Lund 1 were able to 

 demonstrate that the endothelial lining in normal marrow is 

 quite complete, but that in hyperplastic marrows this is prob- 

 ably not the case. They found that the injection mass was not 

 so evenly outlined by the walls of the sinusoids in the hyperplas- 

 tic as in the normal marrows. This difference they explained 

 as due to young red cells which had eroded the endothelial wall 

 by pressure, while, at the same time, these cells were so closely 

 packed together that they prevented the injection mass from 

 extravasating freely into the parenchyma. 



Our concept of the normal structure of the vertebrate bone 

 marrow has been considerably modified by the demonstration by 

 one of us 2 of a very elaborate capillary bed in the marrow of 

 the adult pigeon. These capillaries were first demonstrated by 

 injection, but it was found that they could only be satisfactorily 

 injected after the marrow had been reduced to a very simple 

 hypoplastic state by starvation. The demonstration of a sim- 

 ilar vascular pattern in the mammalian marrow has been very 

 difficult, and yet such a study is essential since, if these capil- 

 laries occur in mammals as well as birds, it will be of the great- 

 est importance to hematology. The vast number of investiga- 

 tions of the bone marrow which have been carried out on anemias 

 produced by various toxic agents, as e.g., Thorium-X and other 

 radioactive substances, have caused too much injury to the 

 marrow to allow analysis of the normal structure. Starvation 

 has so far proved relatively ineffective in the mammal and no 

 other satisfactory method has yet been found for reducing the 

 marrow to the hypoplastic state which has been obtained in the 

 pigeon. 



We have been able, however, by a very simple procedure, to 

 obtain a marrow in the rabbit in which the myelocytic cells are 

 very largely removed and only the developing reds retained; 

 in such a marrow it has been immediately possible to see that 

 there are large numbers of young red cells definitely within 

 capillaries which are not part of the ordinary blood filled sinuses 



i Drinker, Drinker, and Lund, Amer. Journ. Physiol., 1922, lxii, 1. 

 2 Doan, Contributions to Embryology, 1922 xiv, 29, Carnegie Inst, of Wash- 

 ington, Publication No. 277. 



