CARDIAC-LOOP FORMATION IN CHICK 387 
The most conspicuous change in the atrial region is its lateral 
expansion. As early as forty hours the future atrial region is 
dilated so that its transverse diameter is greater than that of 
any other part of the heart tube. From the first the dilation to 
the left is more marked (pl. 3, E, F). When the bulbus is thrown 
against the right side of the atrium in the formation of the car- 
diac loop (pls. 2 and 3, H), it seems to crowd the less developed 
right atrium and retard its development still more. After the 
configuration of the loop has changed so that the bulbus slips 
by the atrium, and crosses the heart at the atrioventricular con- 
striction (pls. 2 and 3, I), the right atrium begins to expand more 
rapidly, but the size of the two atria does not become equalized 
until after the stages here under consideration. 
The first indication of the separation of the atrium into two 
chambers appears in chicks of 29 to 30 somites (53 to 55 hours). 
A longitudinal sulcus develops at this time on the ventrocephalic 
face of the atrium. In a ventral view of the heart this sulcus 
is at first concealed by the truncus and bulbus; but as it becomes 
more clearly marked, its caudal portion can be seen extending 
toward the atrioventricular constriction (pl. 1, J. K. L, 7-a.g.). 
This interatrial groove is an external manifestation of the 
formation of the septum superius. (The septum superius or atri- 
orum of the chick heart corresponds to the septum primum of the 
mammalian heart. In the chick no septum secundum is formed.) 
The formation of the interatrial groove does not appear to be 
dependent on pressure exerted on the atrium by the truncus 
arteriosus. When the groove first appears, an appreciable space 
separates the truncus from the atrium. With further growth, 
however, the truncus appears to sink into the cephalic portion 
of the interatrial groove, and the auricles expand rapidly on 
either side of it. Under these later conditions, the truncus prob- 
ably does play a secondary part in the division of the atrium in 
the sense that it acts as a constricting band on either side of 
which the auricles expand. 
