TAIL-REGIOX OF TOLLSTOTliEMA (dDELLOSTOMa) STOETI. 350' 
Fig. 14. — Transverse section throngli tlie centre of the left einlnyonic 
candal lieart of a 20 nun. Polistotrenui, series A. Its exact position 
is shown hy the line 14 in Fig. 7. 01)serve the direct anastomosis of a 
so-called intersegmental vein with an intersegmental artery and the 
termination of a mesenchymal space (C.) in the vein. This shows that 
an intersegmental vein may take some part in the formation of the 
candal heart. A comparison of the adjacent strnctiires is also of 
interest. X 130, 
[Figs. 15 to 18 are introduced to show the nature of the orifice l>etween 
the ventral veno-lymphatic trunk and the caudal heart, the mode of 
development of the valves guarding this orifice, and something of the 
structure of these vessels in the various stages.] 
Fig. 15. — Transverse section throngh the anterior orifice between the 
embryonic ventral veno-lymphatic trunk and the left embryonic caudal 
heart of a 27 mm. Polistotrema. Note that the endothelium is 
barely differentiated from the mesesenchyme and that no valves have 
appeared, but that there is a proliferation of cells \_v(d. (i)], which may 
be the forerunner of the valves. X 225. 
Fig. 16. — Transverse section throngh the two valves guarding the 
first or anterior orifice l;>etween the ventral veno-lymphatic trunk and 
the left candal heart of a 60 mm. Polistotrema. At a glance it will 
l)e seen that the valves have almost reached adult conditions, that they 
are formed by a proliferation and massing inward and dorsad of the 
mesenchyme between these two structures. At this stage the mesen- 
chyme of the valves has not become differentiated into fibrous con- 
nective tissue lined with endothelium. These valves have made fully 
twice as much advance as the valves guarding the second or posterior 
orifice. It will be seen that the caudal heart is lined with endothelium, 
outside of which the mesenchyme has differentiated into a sort of 
fibrous connective tissue. The musculi cordis caiidalis is well defined 
and fibrillar are making their first appearance in its myoblasts. X 225. 
Figs. 17 and 18 are transverse sections through the valves guard- 
ing the orifice of the ventral veno-lymphatic trunk with the right 
candal heart of the same embryo as Fig. 16, and the two figures are 
from adjoining sections. Both show that the valves guarding this 
orifice are decidedly more embryonic than is the case with the opposite 
or left caudal heart-valves. The valves are here represented as two 
masses of mesenchyme that have grown inward and upward from the 
lateral sides of the orifice, having fused in some places and in others an 
open communication remains. In Fig. 17 the orifice is on the left side, 
in the next section cephalad (Fig. 18) there is no orifice, and in the next 
section cephalad of Fig. 18 the orifice is found to be on the right side. 
X 225. 
