CARDIAC-LOOP FORMATION IN CHICK ou 
than the dorsal, and even at the 9-somite stage its rupture has 
begun in the midcardiac region, although its line of attachment 
to the heart can still be discerned (pl. 1, A). 
In the chick heart the endocardial tubes are less irregular in 
contour and are more completely fused with each other at this 
stage than in the mammalian heart of corresponding age. Fur- 
thermore, the chick endocardium shows no such early foreshad- 
owing of the atrioventricular and the sino-atrial constriction as 
has been observed in mammals (Murray, 719; Schulte, 716; 
Yoshinaga, ’21). In the later stages studied the endocardium 
is of secondary interest, its configuration being determined largely 
by the limitations imposed upon it by the myoepicardium. A\l- 
though the endocardium has been shown in the figures of our 
earliest stages by way of bringing this work into continuity 
with that of other investigators, the later changes in its configura- 
tion have not been followed in detail. 
Between thirty and forty hours of incubation (10 to 18 somites), 
there is a marked dilation of the heart, but its most conspicuous 
change in shape is due to the bending of the entire middle por- 
tion of the heart tube to the right (pls. 1 and 3, AtoE). In this 
process of bending, as indeed in the entire series of changes in- 
volved in loop formation, there is undoubtedly a considerable 
factor of mechanical compulsion. The accompanying graph shows 
how much greater the elongation is in the heart tube itself than 
is the increase, during the same period of time, in the distance 
between the attached cephalic and caudal ends of the heart. 
Under such growth conditions, bending of the heart is inevitable. 
It is quite logical, furthermore, that this bending should be 
lateral because of the impediment offered dorsally by the body 
of the embryo and ventrally by the yolk. Why it should take 
place to the right rather than to the left is not so clear. 
It has been suggested that the bending of the heart to the 
right might be due to the entry of a stronger current of blood 
from the left omphalomesenteric vein than from the right, the 
left vein being conspicuously larger at this stage of development. 
Sabin (17) has shown, that while heart contractions begin in 
the chick as early as the 10-somite stage, the actual circulation 
