THE HISTOLOGY OF THE BLOOD OF LARVA OF LEPIDOSIREN PARADOXA, 461 
hypoblast, and though differmg from the cells lining the gill clefts and pharynx, in 
respect of their limited protoplasmic envelope they have a definitely epithelioid 
appearance. The nuclei are closely packed, are round or oval, and in no single 
instance fissured, horseshoe-shaped or lobed. In the whole gland I have detected 
only one leucocyte—an eosinophile, and therefore a mature variety. The appearance of 
the cells is wholly different from that of any class of the leucocytes; and while these 
elements occur in their thousands round the gut in the spleen and kidneys, and 
scattered in every tissue, only one or two occur in the neighbourhood of the gland. 
The so-called lymphoid transformation of the epithelial cells described in various 
forms by Ko.iiker, Prenant, Maurer, Scuuitzze, Nusspaum and Prymak, and 
BrEaRD cannot, so far, have taken place. Up to this stage, then, the thymus can have 
no share in contributing leucocytes to the blood, unless on the impossible assumption 
that the epithelial cells have become lymphoid, gone off as leucocytes, and become again 
replaced by epithelial cells. 
I hold this observation to be ample proof that the leucocytes in Lepidosiren do not 
originate in the thymus in the larval stages, but I have in preparation a note on the 
Thymus which will bring the matter to a perfectly conclusive issue. 
GENERAL REMARKS. 
The divergence of opinion as to the first origin of the blood is so great that it is 
difficult to reconcile the various accounts. My results are suggestive in this con- 
nection, for they show how both primitive hypoblast and mesenchyme may, under 
certain conditions of development, share in blood formation. As in the later phases 
there is, in the nature of things, a degree of uncertainty, I shall attempt to summarise 
my facts in the strictest terms of accuracy, and then draw together such conclusions 
as | think they may reasonably bear. 
SuMMARY OF Fact. 
(1) The primary corpuscles, the first origin of which I have not studied in detail, are 
all alike in characters. 
(2) At a stage sometime before there is any suspicion of hemoglobin being present, 
there are two classes of corpuscles, one with a distinct circumferential equatorial 
fibrillation of the superficial layers of the protoplasm and without attraction sphere, 
the other without such modification of the protoplasm, but with distinct centrosome 
and sphere. 
(3) In all later stages these two kinds of corpuscles coexist, but at stage 32 there 
is a sudden great increase in the proportion of the active type of cell. 
(4) At stage 30, which is critical, there are (a) corpuscles presenting the features of 
intermediate stages between the large ringed yolk-laden bodies and an oval, disc-shaped, 
