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 duct us 

 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 



