J.EriDOSlREN PAKADOXA. 
85 
latter. The sinii -auricular aperture opeus on the right of 
the ridge and is therefore confined to the right auricle. 
Throughout, the right auricle remains the larger, though 
the left expands rapidly — as does also the left ventricle — 
after the development of the pulmonary vein. 
Division of the Heart into Chambers. — As develop- 
ment proceeds the various chambers of the heart come to 
be demarcated from one another; this is largely brought 
about by a marked disproportion in the rates of growth 
at different parts. For a time growth occurs chiefly in 
Text-fig. 11. 
Sagittal section through the heart at Stage 30. A. Auricle, av. r. 
Auriculo-ventricular ridge becoming continuous with muscular 
tissue in the ventricle. F. Ventricle. 
length and width; a little later, however, the pericardiac 
cavity increases in anteroposterior depth, more especially in 
its ventral part, and this allows the heart, as already 
mentioned, to regain a position approximately parallel wdth, 
instead of transverse to, the long axis of the embryo. In 
fact from this stage till the permanent condition is attained — 
as the liver develops and retreats somewhat towards the tail, 
as the left auricle and ventricle expand with the appearance 
of the pulmonary vein, as the yolk is absorbed and the adult 
shape of the anterior part of the body is reached — a certain 
rotation of the loop takes place. The heart loop gradually 
swings back again in a counter-clockwise direction, as seen 
in a dorsal view, about an axis perpendicular to the long' 
