BLOOD AND VITELLINE VESSELS IN AMPHIBIA 379 
spaces, while other channels are being formed, so that in the tru- 
est sense many of these early vessels are not capillaries because 
they do not yet have endothelium. Whether or not the spaces 
within the embryo which develop into capillaries and vessels, are, 
like some of the early yolk channels, without endothelium, and 
are merely spaces in the connective tissue which later receive a 
lining from vascular endothelium, I cannot tell, but I am inclined 
to believe that many of these early vessels are merely spaces 
between the mesodermal cells. That the lacunae themselves in 
the body of the embryo develop endothelium from the surround- 
ing tissue I am inclined to doubt, and whether the spaces come to 
be lined by the growth from sprouts of earlier vascular areas as 
described by Clark (’09) for lymphatics, or whether vascular 
cells migrate in and gradually form an endothelium, as seems to 
be the case on the yolk at least, will have to be left for other investi- 
gations to decide. 
The history of the development of the vessels in Amphibia may 
be summarized as follows: 
The cells which form the endothelium of blood vessels seem to be 
from mesoderm. Some, at least, are from the rather solid thick- 
ened masses found in connection with the early development of 
the blood. Other cells may be formed from rather isolated 
areas such as described by Mollier and Greil and shown in some of 
the figures in this paper. However formed, these cells probably 
soon penetrate into spaces on the yolk and possibly also into the 
body of the embryo. But on the yolk at least, there seems to be 
a circulation for a time before the endothelium lines all these 
spaces. Perhaps not all of these channels are selected for the 
circulating blood after the establishment of an endothelial lining 
of the vascular system. New blood vessels are apparently formed 
by budding from those formed earlier, in a manner possibly similar 
to the growth of lymphatic vessels in frog larvae. Such vessels 
were seen in the process of formation in the growing edge of the 
body-wall as it comes to enclose the yolk in Desmognathus. Some 
of these fine vessels are kept for blood channels, others, in the 
constant reorganization which is going on in the advancing edge, 
may be much modified or lost. 
