An Early Embryo of Myrmecophaga jubata. 263 
The apex of the heart (Fig. 1) is rotated ventrally, so that the longitudinal 
axis of the heart is almost at right angles to that of the body. The left duct 
of Cuvier is well formed and runs over the surface of the auricles. The 
right is a much shorter and wider vessel, receiving, as it enters the small 
sinus venosus, three large veins from the liver. These all enter the sinus 
venosus at the same level. Two of them form capillaries in the liver 
substance, while the third runs to the ventral side and receives the blood 
from a large vessel draining the gut and from the umbilical veins. The 
Fic. 1.—Heart of embryo of M. jubata, from the ventral side. 
r.d.c., right duct of Cuvier ; 7.d.c., left duct of Cuvier ; p.v.c., posterior vena cava ; 
r.a., right auricle ; /.a., left auricle ; v., ventricle ; c.a., conus arteriosus. 
right umbilical vein is small and ends blindly i in the body wall, while ue 
left is large and is continued through the umbilical cord. 
The right auricle, into which the sinus venosus opens, is much larger 
than the left and is separated from it by an incomplete septum. The right 
auriculo-ventricular valves are beginning to form. The auricles open freely 
into the ventricle whose cavity is as yet undivided. It has thick spongy 
walls. The future division is indicated by a longitudinal groove on the 
surface. The ventricle is twisted slightly so that the right half lies ventral 
to the left. 
The conus arteriosus, with very thick walls, issues from the right side of 
the ventricle and runs forward into a short ventral aorta. It bends sharply 
