EXPLANATION OF PLATES 
Series of chick hearts, showing the formation of the cardiac loop and the prog- 
ress of regional differentiation. The heart contours were drawn directly from 
dissections with the aid of the camera-lucida outlines. In drawing the older 
stages, wax-plate reconstructions were used for working out the relations of 
afferent and efferent vessels and as a check on the configuration of the heart shown 
by the dissections. 
In the stages represented in figures E to I torsion has involved the cardiac re- 
gion of the embryo. Since torsion affects the more cephalic regions first and prog- 
resses caudad, the transverse axis of the body of the embryo is at different in- 
clinations to the yolk at the cephalic, and at the caudal end of the heart. The 
drawings of the ventral and dorsal views are oriented from the frontal plane, and 
those of the dextral views from the sagittal plane of the body, at the level of the 
aortic arches. For this reason the sinus region of the heart appears inclined. 
The relation of the heart to neighboring structures in the embryo is shown in 
the text figures. The lettering of the text figures and plates corresponds 
throughout. 
ABBREVIATIONS 
I-VI, aortic arches I to VI Mes.v., ventral mesocardium 
At., atrium (d. right), (s. left) Myc., cut edge of myoepicardium 
A-V.c., atrioventricular constriction Myc.F., myoepicardial fusion® 
Bul., bulbus cordis S-A.c., sino-atrial constriction 
B-V.c., bulboventricular constriction Sin-at., sino-atrial region (before its 
Cuv.d., duct of Cuvier definite division) 
Endc., endocardium S.V., sinus venosus 
Hep.s., stubs of some of the larger he- V.ao.r., ventral aortic roots 
patic sinusoids Vent., ventricle 
i-a.g., interatrial groove V.O.M., omphalomesenteric veins 
z.v.g., interventricular groove V.O.M.M., fused omphalomesenteric 
Mes.d., dorsal mesocardium veins 
’ During the fourth day there is formed a curious attachment between the 
myoepicardium of the ventral wall of the sinus venosus and the myoepicardium 
of the ventricle (pl. 2, K, L). I have not seen it described elsewhere. From 
work now in progress on later stages of development, I believe this strand be- 
comes a broad fusion and serves as a pathway over which one of the main 
coronary veins from the ventricle reaches the coronary sinus. 
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