Histogenesis of the Blood Platelets. 273 



undoubted, characteristically staining blood platelets in the blood 

 of all of a considerable variety of mammals including the elephant, 

 kangaroo, opposum and camel, and I have found megakaryocytes 

 in the blood-forming organs of all mammals including the opos- 

 sum, which I have examined under satisfactory conditions. The 

 so-called spindle cells or thrombocytes of birds, amphibia, rep- 

 tiles and fishes have been held by some writers to be the morpho- 

 logical equivalents of blood platelets, but my studies of the blood 

 and blood-forming organs of these vertebrates have not led me to 

 accept this view. I would offer the hypothesis that these peculiar 

 corpuscles are rather the homologues of the megakaryocytes than 

 of the detached fragments of their cytoplasm or the blood 

 platelets, and that these two kinds of cell have been differentiated 

 from one and the same type of cell which circulated in the blood 

 of extinct vertebrates. 



This hypothesis is based upon the following considerations: 

 FiEST. As I have already pointed out, the forerunners of the 

 megakaryocytes are at first circulatory cells in the blood of the 

 embryo guinea pig. This fact points to the megakaryocyte as 

 representing a circulatory cell in the ancestry of the mammals. 



Second. The spindle cells or thrombocytes of certain amphi- 

 bian blood have a cytoplasm which stains in the same way as 

 does that of the megakaryocyte; namely, showing a granular 

 red to violet staining endoplasm and a hyaline blue staining ecto- 

 plasm, (see Figs. 18 to 21) . Furthermore these cells in Batrachoceps 

 attenuatus regularly lose their cytoplasm by a pinching off proc- 

 ess and the portions thus detached appear as independent 

 corpuscular elements of the blood with great likeness to blood 

 platelets, for they have the same form and outline as blood plate- 

 lets and the same red to violet staining central portion with vac- 

 uole-like spaces in it and the same hyaline blue marginal portion 

 with the irregular or jagged edge. Figs. 18 to 21 show some of 

 the various forms in which these cells appear and some of the 

 platelet-like detached portions of their cytoplasm as well as some 

 of the phases of the process of detachment. The likeness of these 

 detached portions of the cytoplasm of these cells to blood plate- 

 lets was first pointed out by G. Eisen. 



