ZOOLOGY: C. R. STOCKARD 
561 
to offer the necessary harbor for dividing blood cells, until the red 
bone marrow is the only tissue presenting the proper relationship of 
spaces and vessels, and here alone the erythropoetic function exists to 
supply the red blood cells for the entire body circulation. The red 
blood corpuscles are always produced so as to be delivered into the 
vessels and thus very soon occupy an intra-vascular position and cease 
to divide, while the white blood cells arise and remain for some time 
among the mesenchymal tissue cells in an extra-vascular position. 
11. Lymphocytes and leucocytes, so-called white blood cells, along 
with the invertebrate amoebocytes, are all generalized more or less 
primitive wandering cells, and are almost universally distributed through 
out the metazoa. 
Erythrocytes, red blood corpuscles, are very highly specialized cells 
with a peculiar oxygen carrying function, due to their haemoglobin 
content. In contrast to the universal distribution of the leucocytes, 
the erythrocytes, the red corpuscles, are only found in the vertebrate 
phylum and in a few of the higher invertebrate groups. Yet in these 
invertebrates the oxygen carrying blood cell never presents the typi- 
cally uniform appearance of the vertebrate erythrocyte. The oxygen 
carrying function in many invertebrates is confined to the liquid plasma. 
The typical vascular endothelial cell is widely distributed in the ani- 
mal kingdom and appears to be a simple slightly modified mesenchymal 
cell. 
These three very different types of cells all seem to arise from meso- 
derm — the mesenchyme. Yet the present investigation would indicate 
that each arises from a distinctly separate mesenchymal anlage. The 
erythrocyte, red cell, anlage is localized and perfectly consistent in the 
quality of its production. The lymphocyte and leucocyte, white cell, 
anlage is more diffusely arranged and not definitely localized in any 
particular cells group. The vascular endothelium appears to be formed 
in loco in almost all parts of the embryonic body, and its formation is 
absolutely independent of a circulating fluid or the presence of blood 
cells. 
On the yolk-sac of Fundulus embryos one finds four distinctly differ- 
ent products, red blood corpuscles, endothelium, and two varieties of 
chromatophores, differentiating from the apparently similar wandering 
mesenchymal cells. The environment in which the four types differ- 
entiate is identical as far as is possible to determine, and the only ex- 
planation of their various modes of differentiation is that the original 
mesenchymal cells that wandered out were already of four potentially 
different classes. These differences in potentiaHty within the cells pro- 
