94 HOWELL. [Vol. IV. 



not difficult to recognize tlie cliief stages of karyokinetic divis- 

 ion. When the marrow is preserved in Flemming's solution, 

 and the sections are stained in saffranin, the nuclear figures are 

 very well preserved, and undoubted nucleated red corpuscles, 

 showing the skein, monaster and diaster, can be obtained with- 

 out trouble, as shown in Fig. lo. Nucleated red corpuscles 

 with two nuclei and the cell partially constricted between, — 

 that is, the last step in the process of division, — are especially 

 common. We must admit, then, that the nucleated red corpus- 

 cles have the power of independent multiplication. But this 

 power of reproduction is not unlimited ; and this, it seems to 

 me, is an important fact which has hitherto been overlooked. 

 It is not difficult to determine when the cell has lost its power 

 of reproduction : it is indicated plainly by the appearance of 

 the nucleus. The changes in the structure of the nucleus of 

 the nucleated red corpuscle have been described already in 

 detail, especially the two extremes designated as the mature 

 and immature form of the nucleus. The immature nucleated 

 red corpuscles have a nucleus like that of the erythroblast, 

 preserving a definite reticulum, and, like the erythroblast, it is 

 capable of karyokinetic division. But the offspring or daughter- 

 cells of this form have nuclei belonging to the intermediate class, 

 in which the reticulum is less marked, and the whole nucleus 

 shows a tendency to diffuse staining. These cells are very 

 common in the marrow, and it is probable that they also are 

 capable of multiplication. But sooner or later the offspring 

 of these cells show nuclei with no reticulum at all, and stain- 

 ing diffusely and deeply with the different dyes. This is the 

 mature form, and is further characterized by the deeper color 

 of the haemoglobin in the cell substance. This cell is now 

 ready to lose its nucleus, and become an ordinary red cor- 

 puscle ; and, as far as I can determine, nucleated red cor- 

 puscles which have reached this stage are incapable of any 

 further multiplication. The mature corpuscles are usually 

 smaller than the immature forms, as the successive offspring 

 show a gradual diminution in size both of the nucleus and the 

 cell substance. It is impossible to say how many generations 

 intervene between the youngest nucleated red corpuscle, in 

 which haemoglobin has just appeared, and the mature form, 

 with its peculiar nucleus and greater haemoglobin contents. 



