THE HISTOLOGY OF THE BLOOD OF LARVA OF LEPIDOSIREN PARADOXA, 439 
(mesonephros). The leucocytes, in their several varieties, are coursing in the blood 
stream or wandering among the tissues, especially in the walls of the alimentary 
tract; they are also crowded in the spleen and, as will afterwards be shown, along a 
tract around the kidney. 
Though I have thus divided the history of the blood corpuscles into three phases, 
it is not to be understood that they are sharply marked off from one another, but that 
they overlap and merge into one another imperceptibly. 
PuasE I. 
The Primitive Blood Corpuscles. 
With the actual primary origin of the blood corpuscles, and the development of the 
heart and vessels, this paper will not concern itself, nor will it deal with the general 
question regarding the morphological relations of the blood in terms of the germ layers. 
This will be dealt with by Mr Granam Kerpr himself in a future memoir. 
I begin at a stage, No. 26, when the rudiment of the heart is laid down. In it there 
are free cells, not to be distinguished by any of their characters from the fixed cells 
forming its wall. In the middle line, below the notochord, the aorta appears like a cord 
of cells, identical in general characters with the cells of the developing general mesen- 
chymatous tissue. The same is true for the cardinal vein. 
On the surface of the mass of yolk cells, immediately beneath the fused somatic and 
splanchnic layers of mesepithelium, there are groups of free rounded cells, in spaces 
which later become definite vessels (fig. 1, Pl. I.). 
As I have said, I have not actually studied the origin of these various free rounded 
cells, but from certain incidental observations, and from later phases of the develop- 
ment of the corpuscles, I may express my belief, subject to the revision of Mr GraHam 
Kerr's future researches, that the corpuscles have a multiple origin im situ from the 
general mesenchyme in connection with the developing blood-vessels, and from an 
irregular layer of smaller yolk-laden cells lying beneath the splanchnic mesepithelium, 
on the exact provenance of which I do not wish to express an opinion. I make this 
statement with every reserve, and only make it at all because of its bearing on later 
stages. 
The characters of the primitive blood corpuscles (figs. 1 and 2, Pl. I.) are deter- 
mined by the size and number of the yolk masses in the protoplasm. In fig. 2 is 
represented at one pole of the nucleus an area free of yolk, including a spot I have taken 
for the centrosome, and from it, radiating among the yolk grains, there are delicate 
threads of protoplasm reaching the periphery of the cell. The nucleus is rounded or 
slightly oval, and sometimes shows a notch. The chromatin is collected into rounded 
karyosomes, from which delicate processes ramify to join those of other karyosomes to 
complete the reticulum. 
Ina stage older, No. 27, the appearance of the corpuscles is considerably altered 
