272 James Homer Wright. 



amount of red marrow with consequen^t increase in the total 

 number of megakaryocytes in the body. In so-called myelogenous 

 leukemia the blood platelets are also increased in number, and 

 in the cellular accumulations of this disease megakaryocytes do 

 not seem to be an uncommon finding, although but little atten- 

 tion has been paid to them by pathologists. In view of the enor- 

 mous increase of the marrow cells in this disease it must be obvious 

 that the presence among them of a relatively small proportion of 

 megakaryocytes means a great absolute increase in the number of 

 such cells in the body. 



Furthermore, C. H. Bunting has recently shown experiment- 

 ally on the rabbit that synchronous with or preceding the appear- 

 ance of an increased number of platelets in the blood stream the 

 megakaryocytes are increased in number. 



3. It would seem that the blood platelets do not appear in 

 the embryo before the appearance of the forerunners of the mega- 

 karyocytes. Thus in an embryo guinea pig of about 4.5 mm. in 

 length, I have found, free in the blood vessels, cells of about the 

 size of the nucleated red blood corpuscles which have the char- 

 acteristic staining of the megakaryocyte and differ from it only 

 in being much smaller in size. A study of other embryos shows all 

 grades of transition between this circulatory cell and the typical 

 megakaryocyte. These small megakaryocytes may be seen in 

 the blood vessels in the sections breaking up into typical blood 

 platelets just as do the fully developed cells. (Fig. 17.) On the 

 other hand in a smaller embryo guinea pig I have not found either 

 the small megakaryocytes or blood platelets in the blood. 



It is of interest to note in this connection that some at least 

 of these forerunners of the megakaryocytes seem to be formed by 

 a transformation of endothelial cells of blood vessels, because I 

 have seen one of them apparently forming a part of the endothe- 

 lium of a blood vessel in the yolk sac of a guinea pig embryo. This 

 cell is pictured in Fig. 16. 



4. According to my own and others' observations, bodies that 

 are undoubtedly and obviously blood platelets are found only in 

 the blood of mammals, and mammals are the only creatures that 

 have megakaryocytes in the blood-forming organs. I have found 



