4492 DR THOMAS H. BRYCE ON 
Up to this stage none of the corpuscles contain heemoglobin, as I have been able to 
ascertain, at least for the youngest of the series, by staining with methylene blue and 
eosin ; but in the next following stage, between which and stage 30, however, there is a 
small hiatus, the corpuscles contain hemoglobin, and the megalocytes have disappeared. 
The giantism of the corpuscles can hardly, however, be related to the hemoglobin 
formation, because there are other hemoglobin-free cells in the blood, very different 
in character. These are much smaller cells, and belong to several different categories. 
First, there are elements having all the characters of the later red corpuscles, except 
that they have round nuclei (figs. 8 and 9, Pl. I.), which have precisely the same 
characters as those of the large yolk-laden corpuscles. As all intermediate stages are 
observed between these corpuscles and the large ones, there is reason for believing 
that they are simply the large cells in which all the yolk is used up and the coarsely 
vacuolated protoplasm has been reduced to a fine alveolar condition. To the same 
category, cells with lobed nuclei like that in fig. 15, Pl. I. probably belong. On the 
other hand, there are, second, still smaller elements (fig. 12, Pl. I.), with smaller rounded 
nuclei. These must arise in some other way. They are found in the heart, but 
perhaps more frequently in the cardinal veins and in the vitelline vessels. 
The appearance represented in fig. 16, Pl. I. seems to point to one mode of formation. 
A large yolk-laden cell has apparently divided unequally, the larger moiety remaining 
among the yolk cells, and forming part of the layer of smaller cells still incompletely 
separated from the larger yolk cells, while the smaller portion, consisting merely of a 
nucleus and a small zone of yolk-free protoplasm, is budded off to become a free cell. 
I have also observed one instance suggesting that a multinucleated cell is being 
resolved into a number of elements; but as I have been unable to discover another, | 
cannot speak definitely regarding this possible source of these small corpuscles. That 
these small elements become young erythrocytes is indicated by the occurrence of such 
cells as figured in figs. 13 and 14, Pl. I. 
The first change in the conversion is the appearance of a delicate layer of fibrille 
in the protoplasm, forming an ill-defined rig when the corpuscle is seen on the flat, 
and showing as the group of apparent granules in profile view (fig. 14, Pl. L.). 
Third, still another type of free cell also occurs (fig. 17, Pl. I.) in which the 
nucleus is polymorphic. The protoplasm is exceedingly homogeneous, and shows only 
very indistinctly any reticular or alveolar disposition, while there is an exquisite radial 
sphere and large single centrosome. These cells occur very rarely. They are in 
general aspect the same as the elements of an earlier stage (fig. 10, Pl. I.). I have 
said above that these latter possibly represented the earliest mobile stage of the blood 
cells, or that they were the earliest representatives of the free mobile white corpuscles. 
I have little hesitation in naming such a corpuscle at this stage an early leucocyte, 
both from its characters and because it is arising from the district of the potential 
mesenchyme, in which, in the next phase, the leucocytes begin to appear in large 
numbers. 
