THE HISTOLOGY OF THE BLOOD OF LARVA OF LEPIDOSIREN PARADOXA. 4438 
Passe II. 
As already stated, the alimentary tract up to stage 30 is represented by a solid 
mass of heavily yolk-laden cells, which can only be distinguished into central large 
and peripheral smaller cells. In no part is there any epithelial disposition of these 
primitive hypoblast cells. Immediately after this stage the gut begins to be cut off 
from the mass of yolk cells, at first as a solid cord with a palisade grouping of its 
cells. This cutting off of the alimentary tract is associated with the differentiation of 
a layer of splanchnic mesenchyme round it. I am not here concerned with the details 
of the process, nor with its general significance, but only with its special significance 
regarding the history of the blood. The only point I desire to state is that the 
splanchnic mesenchyme of that part of the alimentary tract from the pharynx to the 
region in which, later, the pancreas rudiment lies, is laid down as a direct derivative of 
the layer of smaller yolk-laden cells which seem simultaneously converted into the 
definitive hypoblast and the mesenchyme. The two layers are at first absolutely 
continuous, the line of demarcation being indicated only by the retention in the 
hypoblast for a longer period of the yolk grains. Later, however, they are sharply 
marked off by the rounded hypoblast elements assuming the form of epithelial cells. 
Up to this point I think the evidence is fairly clear that the blood corpuscles increase 
in number both by direct division and also by new formation. This new formation 
ean only be from two sources—from the somatic mesenchyme as the vessels are formed 
in it, or from the layer of yolk cells lying beneath the splanchnic mesepithelium. Both 
probably before, both almost certainly now, share in contributing to the blood; but as 
the definitive hypoblast and the splanchnic mesenchyme are now differentiated, a new 
phase is inaugurated. 
I shall first enumerate the different types of free cells met with in the blood, and 
then refer to their seats of origin. 
Four stages (— 31, 31+, 32, and 32+) may be taken together for the classification of 
the types, though in referring to their origin I shall have to discriminate between 31 
and 32. As stage 32 was the one selected for the study of the cytological characters of 
the corpuscles, I must refer to the plates published with Part I. of this memoir for 
most of the illustrations. 
Ist. Hrythrocytes. 
All the giant corpuscles have now disappeared from the blood. The corpuscles have 
assumed their definitive disc shape (figs. 1 and 2, Pl. L, Part I.). They now contain 
hemoglobin. Three varieties occur, differing in the characters of the nuclei. The great 
majority have oval nuclei with a very coarse homogeneous chromatin reticulum, which 
takes the orange dye from the mixture of Ehrlich; a certain proportion have round 
nuclei and a relatively small cell body, while many have vesicular nuclei. As the 
TRANS. ROY. SOC. EDIN., VOL. XLI. PART II. (NO. 19). 66 
