THE HISTOLOGY OF THE BLOOD OF LARVA OF LEPIDOSIREN PARADOXA. 445 
nuclear cells vary in size, some being smaller and also showing sometimes a deepish 
eleft (fig. 25, Pl. III.). 
The contrast between the nuclei of the mononuclear cells and the erythroblasts is 
important. It corresponds, in general terms, exactly with the contrast between the 
erythroblast and leucoblast in higher forms, as established by Lowrr and then Denys, 
and more recently, still further elucidated by PapPpENHEIM.* 
Ath. Small Mononuclear Cells. 
These have a small round or notched nucleus, with a relatively dense disposition of 
the chromatin and a narrow zone of delicate hyaline basophile protoplasm. 
5th. Leucocytes proper. 
These are found sparingly in the blood, but crowded in the spaces of the mesen- 
chyme. Outside the blood stream they are simply free wandering mesenchyme cells. 
They vary greatly in size; the nucleus shows all degrees of polymorphism; there is 
always a large centrosome and sphere; the protoplasm is either hyaline and basophile, 
or granular in various degrees, from a few scattered extremely small granules to larger 
bodies filling the whole cytoplasm, when the cells have all the characters of eosinophile 
leucocytes. In a few instances the granules have a basophile reaction, taking the blue 
instead of the eosin from a methylene blue and eosin stain. 
In the succeeding section the origin and inter-relation of these various elements will 
be considered. 
The four stages under consideration present a gradual unfolding of the conditions 
which characterise the second phase in the development of the blood. 
The disposition of the splanchnic mesenchyme at stage 31, may be gathered from a 
section, passing through the liver just behind the pharynx and heart. The solid cord of 
cells which represents the future stomach is seen passing down on the left of the liver 
to be continuous below with the still undifferentiated yolk cells. Surrounding it, and 
passing below over the surface of the yolk, is a very cellular tissue, which also here 
surrounds the solid rudiment of the bile duct, and passes along it into the liver. 
This mesenchyme is a loosely arranged tissue, everywhere permeated by irregular 
Spaces containing red blood corpuscles, but without definite endothelial walls. 
The component cells (fig. 2, Pl. II.) are both free and fixed. The latter seem to be 
elongating to form spindle-shaped elements, while the former appear to lie free in the 
intercellular spaces. These free cells are already of more than one variety. The 
majority are large mononuclear cells, but there are also cells with metamorphosed nuclei, 
and some even are distinctly granular. The layer is directly continuous with the under- 
lying hypoblast, in which mitotic figures are observed. 
* Archiv f. path. Anat., vol. 151, 1898. PappENHEIM gives a very extensive bibliography. His theses, summarised 
at the close of his paper, agree in respect of these blood cells closely with those of this paper, 
