444 DR THOMAS H. BRYCE ON 
vesicular condition of the nucleus is characteristic of the great mass of the erythrocytes 
in all the later stages, we may take it that the corpuscles with this character are the 
mature elements. I may here remark that after stage 32 the number of dividing 
erythrocytes gradually diminishes, until in later phases they are very rare except in the 
spleen, where it is only the young erythrocytes which are dividing. I believe that the 
mature erythrocyte is probably incapable of division. Those with the oval nuclei 
showing a reticulum are therefore young corpuscles, and at the stages under considera- 
tion they are in very active division. Those with round nuclei are probably transi- 
tional forms between the erythroblast and the young erythrocyte; they are also in 
active division. 
2nd. Hrythroblasts. 
These are small cells with a round or oval nucleus and small cell body, which, 
though basophile, has a delicate concentric fibrillation. Such cells are represented in 
figs. 18, 19, Pl. I., and fig. 24, Pl. ILI). There are two stages in the fibrillation of 
the protoplasm. The cell drawn in fig. 24, Pl. III. has only a narrow zone of proto- 
plasm and the fibrillation is extremely fait. In fig. 18 there is a distinct marginal 
band, with a zone of apparently alveolar protoplasm round the nucleus; when seen in 
profile section (fig. 19, Pl. I.) the band shows at the extremities of the oval body as a 
series of fine dots. 
The characters of the nucleus are very important. Compared with the mononuclear 
cells of this stage and with the young red cells of the blood at stage 30, it is seen that 
the chromatin nucleoli are larger, massed closer together, and the intervening reticulum 
is coarser, so that the nucleus stains more deeply. These cells apparently correspond to 
what Gicxio-Tos * calls ‘thrombocytes,’ after DEKHUYZEN. He regards them, not as 
stages of the red corpuscles, but as special elements derived from leucoblasts, and distin- 
guished from the erythroblasts by several characters, one of which is the concentric 
disposition of the fibrillee of the protoplasm, while in the erythroblasts the filaments are 
radially arranged. It is just the concentric fibrillation of the protoplasm, leading up 
to the fibrillar band, which determines in this case the nature of these cells. 
3rd. Large Mononuclear Cells. 
I have given this name to these elements because of the large simple nucleus, which 
is either round or notched (figs. 24, 25, Pl. IIL, Part L.). It is distinguished from the 
erythroblast by the more purely basophiie reaction of the protoplasm, which is never 
fibrillar, showing only an extremely delicate reticulum. The nucleus has its chromatin 
nucleoli rather widely scattered ; they are relatively small, and the intervening reticulum 
is very delicate and filamentous. It is to be specially noticed that all the corpuscles 
up to stage 30, when the hemoglobin appears, have nuclei of this type. The mono- 
* Arch, [tal. de Biol., T, xxix., 1898, and Mem. a. R. Accad. delle Sc. di Torino, s. ii. t. xvii. 
