TAIL-liEGJON (JF TOLIs'I'OTK KM A (r.DKLLOSTOM a) STOUTI. 333 
iutersegmeiital vein that liad been intercepted in its growth 
by the crossing of a ventral intersegmental artery^ or it may 
liave been oidy a capillary, or possibly a caudal lieart 
mesenchymal cavity that had gained connection both with the 
caudal vein and an intersegmental artery. At any rate, 
granting it to be an embryonic intersegmental vein, it could 
have but little to do, relatively, with the formation of the 
left caudal heart. The mesenchymal space (Fig. 14, C.) 
opening into the so-called embryonic intersegmental vein has 
every indication of -being an isolated caudal heart mesen- 
chymal space that has acquired connection with the so-called 
embryonic intersegmental vein, rather than being a sprout 
from the vein. It will be seen, then, that the intersegmental 
veins or capillaries, which, according to Lewis, Baetjer, 
Huntington, and Miller, take such an important part in the 
formation of the lymphatic hearts in mammals, birds, and 
reptiles, takes but a very insignificant, if any, part in the 
fortnation of the caudal hearts in Polistotrema. 
Both of the reconstructions (3 and 7 demonstrate that the 
left embryonic caudal heart has at this stage received no 
communication from the ventral veno-lymphatic trunk. In 
series B, reconstruction 6, the ventral veno-lymphatic trunk 
is represented by at least one large mesenchymal cavity 
[L V. V. d); which is about to gain connection with the embry- 
onic caudal heart throuo*h the breakino* down of the thin wall 
o o 
separating them. Also but little has been accomplished 
toward the differentiation of an endothelial lining, other than 
the thickening of the cell-processes lining the cavity and the 
slight elongation of some of the cells ; it would be extremely 
ditticult, if not impossible, to identify one of the typical dorsal 
or ventral border cells of the heart cavity from a typical 
mesenchymal cell. There is a marked massing of the mesen- 
chyme to form the connective-tissue layer of the heart, but no 
differentiation of mesenchyme into connective tissue. 
