I20 



HOWELL. [Vol. IV. 



The large nucleated giant cell, or megakaryocyte, unlike the 

 multinucleated form, is found, and found abundantly, in the 

 midst of the red marrow filling up the ends of the long bones 

 and the spaces between the trabeculae of the spongy bone. It 

 does not lie upon the spicules of bone or cartilage, but away 

 from them, surrounded by marrow cells, nucleated red corpus- 

 cles, blood corpuscles, and the other cells characteristic of 

 marrow. It appears in the marrow with its first formation, 

 as shown by cross and longitudinal sections of the femur in an 

 embryo cat 9 cms. long, and is then surrounded by the ele- 

 ments of the marrow. Throughout the rest of the animal's 

 life it can be discovered in sections or teased specimens of the 

 red marrow. It is evidently a peculiar kind of cell, which has 

 some definite function to perform. Not only is it found in the 

 marrow, but throughout embryonic life it is met with in abund- 

 ance in the liver and the spleen as long as these organs have 

 any distinct connection vs^ith the production of red blood cor- 

 puscles ; and it occurs more abundantly, the more active the 

 blood-forming function of the organ. In histological structure 

 it is the same in the embryonic liver and spleen as in the 

 adult marrow. This fact has been stated by others (Foa and 

 Salvioli [i], etc.), and is perfectly evident to any one who will 

 take the trouble to look. It is certain, therefore, that the 

 function of this cell is the same in the embryo spleen or liver 

 as in the adult marrow. That it is not simply one stage in 

 the life of the multinucleated giant cell seems to be demon- 

 strated by the fact that typical multinucleated cells are never 

 found in the mass of red marrow, in the cat at least, lying in 

 the cavity of the long bones, nor in the embryonic liver or 

 spleen, though the megakaryocytes are so numerous. 



The structure of the megakaryocytes has been clearly de- 

 scribed by a number of observers, especially by Arnold (3), 

 They are giant cells, each with a huge nucleus. The body of 

 the cell is finely granular, and shows no special peculiarities in 

 structure. It is interesting to note that it does not possess the 

 power of amoeboid contraction which is so marked in many of 

 the marrow cells, especially those with elongated nuclei. The 

 nucleus is the characteristic part of the cell, but varies consid- 

 erably in size, complexity, and minute structure (Fig. 11, a, b, 

 c, d). Frequently it is crescent-shaped, or even makes a ring ; 



