Q2 ■ HOWELL. [Vol. IV. 



marrow. Of course, there may be a difference in structure in 

 these apparently similar cells, according to the fate which 

 befalls them ; but, if so, it is not apparent as a morphological 

 characteristic. The ordinary marrow cell, as has been de- 

 scribed, is characterized histologically by its vesicular nucleus, 

 which has one or more prominent nucleoli and a scanty reticu- 

 lum. In the forms intermediate between this and the erythro- 

 blast we find that the nucleoli, or nucleolar matter, becomes 

 scattered throughout the nucleus in the form of smaller gran- 

 ules ; while the reticulum becomes more pronounced, and unites 

 with the granules to give the characteristic nucleus of the 

 erythroblast. While in the latter cell, therefore, we have no 

 distinct nucleoli, we do have a number of small granules of 

 nucleolar material situated at the nodal points of the reticulum. 

 This transformation from a marrow cell to an erythroblast does 

 not take place by gradual changes going on in one cell, but 

 makes its appearance more or less gradually in successive gen- 

 erations. The original marrow or embryonic cell multiplies 

 by indirect division ; and the daughter-cells, instead of having 

 a single large nucleolus, have several smaller ones scattered 

 throughout the nucleus and connected with its reticulum, 

 showing thus an approximation to the structure of the erythro- 

 blast, the cells also being of a smaller size. These cells in 

 turn multiply ; and their offspring either become erythroblasts 

 or at least resemble them more closely. One cannot say how 

 many generations — one or more — are necessary for the 

 change. All that can be observed is that between the large 

 embryonic cells and the smaller erythroblasts there are found 

 cells intermediate in size and in the structure of the nucleus ; 

 and it seems more reasonable to suppose that these changes 

 take place after successive divisions during the re-formation of 

 the nucleus from the chromatin filaments rather than from a 

 process of condensation and alteration going on in each cell. 

 Denys (14) has found in the marrow of birds that the erythro- 

 blasts are separated from the other elements of the marrow, 

 and lie in cords, which are in reality a part of the vascular 

 system of the marrow. I have described a similar arrange- 

 ment in the liver of the young embryo cat. But if such an 

 arrangement of the erythroblasts exists in the marrow of the 

 cat, it is certainly very much obscured, as repeated examina- 



