FROM TWELVE TO THIRTY-SIX SOMITES 201 
arches at the anterior end and a point little behind the entrance 
of the ducts of Cuvier into the heart at the posterior end. 
Two main changes characterize the development of the heart 
in the period under consideration: (1) folding of the cardiac tube 
and (2) differentiation of its walls in successive regions to form 
the four primary chambers of the heart, viz. (from behind for- 
wards), the sinus venosus, the auricular division (atrium), the 
ventricular division and the bulbus arteriosus. 
The folding of the heart is caused by the rapid growth between 
its anterior and posterior fixed ends, and the places of folding 
are determined largely by differences in the structure of the walls 
at various places. The folding begins by a curvature to the 
right, and this proceeds until the tube has an approximately 
semicircular curvature (Fig. 72). At a certain place in the 
curved tube a very pronounced posterior projection takes place 
(Figs. 73 and 74), and at the same time this bent portion turns 
ventrally; the apex of the bend represents the future apex of the 
ventricles. The continuation of these two directions of folding 
then brings the ventricular division of the heart immediately 
beneath the sinu-auricular division which is attached dorsally 
by the somato-cardiac connections; further continuation brings 
the apex of the heart a little behind the auricular portion (Figs. 
85, 87, 88, 93, 99). During all this period the distance between 
the two fixed ends has remained practically constant. 
During the process of folding, constrictions have arisen 
between successive portions of the cardiac tube, owing to expan- 
sion of intervening portions, and thus at the stage of seventy-two 
hours the heart shows the following divisions and form. From 
the dorsal surface (in a dissection, Fig. 116) one sees (1) the sinus 
venosus, broad behind and narrow in front where it joins the 
auricular division; it receives three veins: (a) the large ductus 
venosus, appearing as a direct posterior continuation of the sinus, 
and separated from it by only a slight constriction; and (b and c) 
the right and left ducts of Cuvier entering the sinus laterally 
and dorsally near its enlarged posterior end; (2) the sinus enters 
the atrium through the dorsal wall; the atrium shows two lateral 
expansions, the future auricles, of which the left is much the 
more expanded at this time; the sinus appears partly sunk in 
the right auricle. (3) Only the right limb of the ventricular 
loop is visible from the dorsal surface at this time, and is separated 
