104 EARLY EMBRYOLOGY OF THE CHICK 



visceral arches and clefts are ordinarily designated by their 

 post-oral numbers only. 



There are other structures which are just beginning to be 

 differentiated in the pharyngeal region and fore-gut of embryos 

 of this stage, but it seems better to consider them in connection 

 with later stages when their significance will be more readily 

 grasped. 



V. THE CIRCULATORY SYSTEM 



The Heart. In embryos of 30 to 40 hours incubation we 

 traced the expansion of the heart till it was bent to the right of 

 the embryo In the form of a U-shaped tube (Figs. 19, 21, 23). 

 The disappearance of the dorsal mesocardium except at its 

 posterior end, leaves the mid-region of the heart lying unat- 

 tached and extending to the right, into the pericardial region of 

 the ccelom. The heart is fixed with reference to the body of the 

 embryo at its cephalic end where the ventral aortic roots lie 

 embedded beneath the floor of the pharynx, and caudally in the 

 sinus region where it is attached by the omphalomesenteric 

 veins, by the ducts of Cuvier, and by the persistent portion of 

 the dorsal mesocardium. 



During the period between 30 and 55 hours of incubation the 

 heart itself is growing more rapidly than is the body of the 

 embryo in the region where the heart lies. Since its cephalic 

 and caudal ends are fixed, the unattached mid-region of the 

 heart becomes at first U-shaped and then twisted on itself to 

 form a loop. The atrial region of the heart is forced somewhat 

 to the left, and the conus region is thrown across the atrial 

 region by being twisted to the right and dorsally. The ven- 

 tricular region constitutes the loop proper (Cf. Figs. 22, 29 and 

 34). This twisting process reverses the original cephalo- 

 caudal relations of the atrial and ventricular regions. Before 

 the twisting, the atrial region of the heart was caudal to the 

 ventricular region as it is in the adult fish heart. In the twist- 

 ing of the heart the atrial region, by reason of its association 

 with the fixed sinus region of the heart, undergoes relatively 

 little change in position. The ventricular region is carried over 

 the dextral side of the atrium and comes to lie caudal to it, thus 

 arriving in the relative position it occupies in the adult heart. 



The bending and subsequent twisting of the heart lead toward 



