STRUCTURE OF FOUR-DAY CHICKS 141 



the impediment offered to its dorsal bending by the body of the 

 embryo, and to its ventral bending by the yolk. 



The lateral bending of the heart attains its greatest extent 

 at about 40 hours of incubation. At this stage torsion of the 

 body of the embryo changes the mechanical limitations in the 

 heart region. As the embryo comes to lie on its left side the 

 heart is no longer pressed against the yolk (Cf. Figs. 21 and 29). 

 As a result the bend begins to swing somewhat ventrad and lies 

 less closely against the body of the embryo (Figs. 49 and 50, 

 E,F). 



At about this stage of development a new factor affects the 

 changes in the shape of the heart. The closed part of the 

 U-shaped bend is forced caudad and at the same time becomes 

 twisted on itself to form a loop (Figs. 49, F-I and 50, F-I). 

 In the formation of the loop the atrial region is forced slightly to 

 the left (i.e., toward the yolk) and the conus is thrown across the 

 atrial region by being bent to the right (i.e., away from the yolk) 

 and then caudad. The ventricular region constitutes the closed 

 end of the loop. This twisting process reverses the original 

 cephalo-caudal relations of the atrial and ventricular regions. 

 The atrial region which was at first caudal to the ventricle now 

 lies cephalic to it as in the adult heart. 



The atrial region and the ventricular region which formerly 

 were continuous without any line of demarcation, are by this time 

 beginning to be marked off from each other by a constriction 

 (Fig. 49, /, a.v.). As both the atrium and the ventricle be- 

 come enlarged, this constriction is accentuated (Fig. 49, L, a. v.). 

 The constricted region is now termed the atrio-ventricular 

 canal. 



During the fourth day the bulbo-conus arteriosus becomes 

 closely applied to the ventral surface of the atrium. As the 

 atrium grows it tends to expand on either side of the depression 

 made in it by the pressure of the bulbo-conus (Figs. 49, J-L 

 and 50 J-L). These lateral expansions of the atrium are the 

 first indication of the division of the atrium into right and left 

 chambers which are later completely separated from each 

 other. At the same time a slight longitudinal groove appears 

 in the surface of the ventricle (Fig. 49, L, i.v.) which indicates 

 the beginning of the separation of the ventricle into right and 

 left chambers. The division of the bulbo-conus to form the 



