PLATELETS 17 



appearances. They do not consist simply of a mass of faintly 

 basophilic cytoplasm in the middle of which lies a collection of 

 azurophilic granules, which is the character of the normal platelet. 

 They are apparently composed of several such platelets connected 

 together ; in other words, their microscopical characters strongly 

 suggest that they are in reality small pieces of the cytoplasm of 

 the megaokaryocyte broken off en masse instead of in smaller 

 fragments. The impression obtained is as of deficient differentia- 

 tion of the megakaryocytic cytoplasm prior to the protrusion of 

 the pseudopodium, so that separation has occurred too soon. 

 Certainly these giant platelets differ in no respect other than size 

 from the cytoplasm of the giant cells of the bone marrow. There 

 is, however, yet further evidence of the accuracy of Wright's 

 interj^retation of his microscopical preparations. In some cases 

 of myeloid leuksemia there are found parts of nuclei in the peri- 

 pheral blood. These pieces have the characters of the nuclei of 

 megakaryocytes, but of course are much smaller. Attached to 

 the nuclear fragment are always seen a number of irregular 

 structures having all the characters of blood platelets. Lastly, 

 histological examination of the lungs often reveals the presence of 

 whole megakaryocytes lying, as emboli, in the pulmonary capil- 

 laries ; here the cytoplasm still shows the arrangement and 

 tinctorial characters of blood platelets. It seems, therefore, 

 quite justifiable to regard the blood platelet as being one of the 

 few structures of which the developmental history is accurately 

 known, although the same cannot be said of the origin of the 

 megakaryocyte itself. It is commonly supposed that these large 

 cells, which may be as much as forty microns in diameter, are 

 derived from the reticulo-endothelium by simple increase in size, 

 but this is far from being certain. It is also known that the cell, 

 when young, is actively phagocytic ; a power that is lost by the 

 older forms. Pari passu with this alteration of functional ability 

 there proceed certain histological changes. Thus the young forms 

 contain many mitochondria of thread-like appearance, whilst a 

 skein of Golgi apparatus lies near the centrosomes ; in older forms 

 mitochondria are less numerous, and the Golgi system can usually 

 not be seen. 



