ZOOLOGY: C. R. STOCKARD 
559 
5. A definite mass of cells characteristic of the fish embryo is located 
in the posterior half of the body between the notochord and the gut 
and extends well into the tail region. This so-called 'intermediate 
cell mass' is the intra-embryonic red blood cell anlage in many of the 
species. 
The peripheral cells of the mass as claimed by Swaen and Brachet, 
or the mesenchyme about the mass, Sobotta, form a vascular endothelium 
which encloses the central early blood corpuscles. In individuals with- 
out a circulation the erythroblasts arise in a normal manner in this 
centrally located position and become erythrocytes or red blood cor- 
puscles filled with haemoglobin. Typical vascular endothelium com- 
pletely surrounds the erythrocytes which, instead of being swept away 
as usual by the circulating current, remain in their place of origin. All 
of the early blood forming cells of this intermediate mass give rise to 
red corpuscles and never to white blood cells. 
6. Contrary to the opinion of most recent observers on blood develop- 
ment in Teleosts, the Fundulus embryos both with and without a circu- 
lation, possess blood islands on the posterior and ventral portions of 
the yolk-sac. These blood islands are formed by wandering mesen- 
chymal cells which migrate out from the caudal region of the embryo. 
They represent all that remains on the yolk-sac of the peripheral meso- 
derm in the Teleosts and probably wander away from mesoderm related 
to that of the intermediate cell mass. The intermediate cell mass may 
possibly represent the bulk of the peripheral mesoderm which is here 
included within the embryonic body, while in other meroblastic eggs 
it is spread out posteriorly over the yolk. The only mesodermal ele- 
ments of the yolk-sac in Fundulus are the independent wandering 
mesenchyme cells, some of which group themselves to form the blood 
islands, while others give rise to the yolk vessel endothelium, and still 
other wandering cells develop into the chromatophores. 
7. The non-circulating red-blood corpuscles within the embryo remain 
in a fully developed condition for eight or ten days and then undergo 
degeneration. In an old embryo of sixteen days it is sometimes found 
that very few of the corpuscles in the intermediate mass are still present 
and these are degenerate. The vascular endothelium has been lost 
and numerous mesenchyme cells have wandered in to lie among the 
corpuscles. 
On the yolk-sac the corpuscles no doubt have a better oxygen supply 
and here they maintain their color longer, but finally also present a 
degenerate appearance with small densely staining nuclei and cell bodies 
much reduced in size. 
