346 WILLIAM A. HILTON 
becomes thickened and may be followed forward to the region 
of the liver. This forms the blood and becomes transformed into 
the vitelline vein in later stages. Although most of the blood 
would thus be of mesodermiec origin, it was impossible to be sure 
that all of the early cells came from a thickening of this germ 
layer, because a certain small number of them seemed to be added 
from the entoderm. In Bufo the blood seems to come almost 
entirely from cells of the yolk mass. However he regards these, 
in a way, as mesodermie cells which have not yet separated from 
the others. 
Marcinowski (’06), studied Bufo and Siredon. The results of 
his work in some respects resemble those of Muthmann, Mollier, 
and Greil. The heart develops from cells which migrate from the 
mesoderm into a little space just in front of the liver. These 
come from the mesoderm on each side of the middle line and not 
from a mid-ventral thickening. In one of his figures he shows 
the forming heart endothelium at an early stage, with two little 
cavities in the mass. He recognizes the ventral keel of entoderm 
which so many observers apparently have confused with the 
heart anlage, and shows very clearly in his diagrams how the con- 
fusion could have arisen. He, like Muthmann (’04), regards this 
as a part of the thyreoid anlage. The vascular endothelium he 
regards as formed from secondary mesenchyme. * The ectoderm 
is shown to furnish some of the primary mesenchyme. Wan- 
dering cells which give rise to endothelium become localized in 
various places. All the larger vessels are developed as isolated 
areas which secondarily come into union. Vascular tissues are 
developed from the sclerotomes and mid-ventral mesoderm. This 
last area gives rise to the blood. ‘The first blood vessels are solid 
and may or may not communicate with the spaces in the mesen- 
chyme or those between connective tissue cells. The endothelium 
is continuous with the spaces at the time the blood corpuscles 
come into the circulation. 
Greil (08), traces the origin of the blood and vascular endo- 
thelium in Ceratodus, Amphibia and other vertebrates. He recog- 
nizes in early stages, at least approximately, the cell areas which 
