CARDIAC-LOOP FORMATION IN CHICK 383 
embryos represented in figure 1, G and H. In the embryos I 
have studied this condition seems to be correlated with delayed 
flexion rather than with any abnormality of the heart tube. It is 
probably to be regarded as within the limits of normal 
variability. 
In the ventral views of the heart it can be seen that as the 
process of loop formation progresses, the extension of the heart 
to the right is diminished, and that the loop as it is formed swings 
not only ventrad, as has been mentioned above, but also dis- 
tinctly toward the sagittal plane (pl. 1, G, H, and I). This 
change in position may well be due to the fact that by this stage 
the body at the cardiac level has completed its torsion and lies 
on its left side, so that the heart is no longer prevented by the 
yolk from expanding midventralward. 
The heart in the stage when the cardiac loop is first definitely 
formed (i.e., in embryos of about 25 somites) has been described 
by many investigators as ‘S-shaped.’ Neither the dorsal (pl. 
3, H) nor the lateral (pl. 2, H) nor the dextrodorsal view of 
the heart as seen in the ordinary whole-mount (fig. 1, H) 
can be characterized as ‘S-shaped.’ Only when a heart model 
or a heart freed by dissection is rotated, so that a direct ventral 
view is obtained, does the ‘S-shaped’ configuration become 
recognizable (pl. 1, H). 
At the close of the second day of incubation, the cranial flexure 
of the embryo is developing extremely rapidly (fig. 1, G, H, I). 
As the anterior part of the head is bent caudad, it begins to crowd 
the heart loop. As a result the ventricular bend of the loop 
moves at first caudad and then dorsad (fig. 1, HtoL). Prior 
to the formation of the cardiac loop and its dorsocaudal bending, 
the ventricular portion of the heart is cephalic to the atrium, in 
the primitive vertebrate position. The bending of the loop 
brings the ventricle caudal to the atrium in approximately the 
definitive relationship characteristic for adult sauropsida. 
