384 BRADLEY M. PATTEN 
THE REGIONAL DIFFERENTIATION OF THE HEART 
Certain text-book diagrams of the chick heart show bulbar, 
ventricular, and atrial regions, separated by conspicuous con- 
strictions while the heart is still in the straight tubular stage. 
Although perhaps suggestions of such constrictions are to be 
detected at this early stage of development, I have not been able to 
satisfy myself as to their definite appearance until the heart is 
well bent to the right, and they do not appear at all conspicuously 
until nearly forty hours of incubation (16 to 18 somites). In 
the heart of a chick of 20 somites, the bulboventricular constric- 
tion, previously but vaguely discernible, has become quite definite 
(pl. 1, F). The atrioventricular constriction is also well marked 
by this time (pl. 2, F). The sinus venosus exists rather as the 
place of confluence of the omphalomesenteric veins with each 
other and with the atrium than as a definite division of the heart. 
Nevertheless, the sino-atrial boundary may be said to be fore- 
shadowed by an increased conspicuousness of the grooves formed 
on either side where the omphalomesenteric veins enter the 
heart at an obtuse angle to it (pl.3, F). The apparent deepening 
of these lateral grooves is, however, due rather to expansion of 
the atrium than to any actual constriction in this region. There 
is as yet no demarcation between sinus and atrium dorsally, and 
no caudal line of demarcation between the sinus and the omphalo- 
mesenteric veins. 
For convenience in description, the heart at this stage can 
best be compared to a U with its upright limbs attached to the 
body of the embryo (fig. 1, F). The ventricle, definitely marked 
off both cephalically and caudally by constrictions, constitutes 
the bend of the U. ‘The bulbotruncus portion of the heart tube 
constitutes the cephalic limb of the U, which is attached to the 
body by the aortic roots. The sino-atrial region constitutes the 
caudal limb of the U, which is attached by the omphalomesen- 
teric veins, the ducts of Cuvier, and the remaining part of the 
dorsal mesocardium. 
The early changes in the bulboventricular portion of the heart 
are already so well known that they require but e, brief summary. 
A te are 
