No. 1.] BLOOD CORPUSCLES. 



91 



forming organs, that it multiplies by indirect division, — divisio 

 per fila, — and is not derived from any other element of the 

 marrow, Osier (28) and Osbratzow (35) think that it develops 

 from naked nuclei found in the marrow, and Foa and Salvioli 

 believe that it is constricted off from the giant cells. The 

 theory of Lowit is the best supported by observations and ex- 

 periments, and has met with most corroboration. While I 

 with others before and after Lowit have satisfied myself of 

 the existence of the erythroblasts, I cannot agree with him 

 that they are not derived from other simpler cells found in 

 the marrow. 



In sections and teased specimens of the liver of the embryo 

 and of the marrow of the embryo and adult, I have obtained 

 evidence to show that the erythroblasts are derived from cells 

 of the marrow similar in structure to the ordinary marrow 

 cells ; that is, large cells with oval vesicular nucleus and a 

 faintly granular protoplasm. Drawings intended to illustrate 

 the way in which these cells give rise to the erythroblasts are 

 given in Fig. 11. The marrow cells themselves have the char- 

 acteristics of embryonic cells ; and those from which the ery- 

 throblasts are derived are undoubtedly descendants, but little 

 if any changed, of the original mesoblastic cells from which 

 the marrow is formed. In the embryonic liver, as well as 

 in the embryonic marrow, these cells are found, together 

 with the transitional stages to the typical erythroblast. This 

 derivation is particularly well marked in the developing blood- 

 vessels of the liver of the young embryo. As I have already 

 said, these vessels consist of a mass of cells destined to become 

 red corpuscles ; and some of them are typical erythroblast, 

 while others are of the character of the marrow cells or corre- 

 spond to what Lowit calls leucoblasts, and others still repre- 

 sent intermediate stages. None of these cells can be regarded 

 as leucoblasts, according to the definition of Lowit, since at 

 this time no typical leucocytes are found in the circulating 

 blood. The embryonic cell from which the erythroblast is 

 derived is found in the marrow of the adult as an ordinary 

 marrow cell. In fact, the marrow cells seem to be undiffer- 

 entiated cells, like the cells of the original mesoblast ; and, 

 while some may change to erythroblasts, others become loaded 

 with coarse granules or develop into the fat cells of the yellow 



