THE HISTOLOGY OF THE BLOOD OF LARVA OF ZEPIDOSIREN PARADOXA. 453 
in the blood stream. From the evidence available, it seems to me justifiable to 
conclude for this stage— 
Ist. That new elements are being added to the blood stream. These belong either 
to the erythrocyte or leucocyte series. 
2nd. In regard to the leucocyte series, all the cells free in the blood which can be 
ealled leucocytes are derivatives of the mesenchyme; certainly of the splanchnic, 
almost certainly of the nephric tract of somatic mesenchyme. 
._ 3rd. In regard to the erythrocyte series—the erythroblast without heemoglobin is 
derived from an intravascular ‘mononuclear element,’ which is in turn derived from 
the mesenchyme, probably from both splanchnic and somatic mesenchyme. 
Ath. The two series arise from a common mother cell—the mesenchyme cell. The 
ehanges which convert the mother cell into an erythrocyte take place in the blood 
stream, while its metamorphosis into a leucocyte is brought about in situ, in the spaces 
of the mesenchyme outside the blood stream. 
Before leaving this stage I must refer to the character of the granulation in the 
oranular leucocytes. In some instances, as I have already stated, these cells have all 
the characters of eosinophile cells. It would take me out of my direct way to go into 
the general question of the meaning of the granulation of leucocytes; and I would here 
merely remark, in regard to these particular cells, that the granulation may be merely 
yolk material in fine division, for I have observed precisely similar eosinophile granules 
in the yolk cells, and the yolk itself has strong affinity for eosin. At this stage I find 
the granular cells crowded together only in the splanchnic mesenchyme and in the 
tract of the somatic mesenchyme so frequently alluded to. It at once suggests itself, 
in the case of the cells near the yolk, that this may be the source of the material which 
constitutes the granulation ; and in regard to those in the neighbourhood of the nephric 
duct, it is worth mentioning that the cells forming the wall of that duct are filled with 
yolk grains long after they have disappeared in the neighbouring tissues. It seems to 
me not impossible that these early leucocytes may be concerned in the distribution of 
yolk food. Were this the case, the granules would necessarily differ from those of the 
leucocytes of later stages, but I see no inherent improbability in the suggestion, which, 
if well founded, has a bearing on the nature of granulation in leucocytes generally. 
Puase III. 
After the second phase, as I have ventured to define it, is fully established at stage 
82 or 33, the general conditions are maintained for a time, during which the histogenesis 
of the spleen is gradually accomplished and the atrophy of the pronephros completed. 
It is with the complete differentiation of the spleen as a hemopoietic organ that what 
I call the 3rd phase in the history of the blood is established, and the renewal of the 
blood corpuscles is confined to that organ, and to the pseudo-lymphoid tissue round the 
kidneys, and possibly also round the gut. It is presumably the adult state of things 
