566 THE HUMAN EMBKYO. 



In the youngest human embryos, as in the corresponding 

 stages of the rabbit, the heart consists of two symmetrical and 

 perfectly distinct halves. On the thirteenth day (Fig. 179, R) 

 the heart is present as a pair of straight tubes lying along the- 

 sides of the anterior end of the embryo, between the neural 

 folds and the yolk-sac, and in connection at their hinder ends 

 with the vessels which return the blood from the yolk-sac. 



At a slightly later stage (Fig. 185, R), the two halves of the 

 heart have united to form a single tube, which is already twisted 

 on itself. 



By the fifteenth day (Figs. 197 and 232), the heart has. 

 advanced considerably in development, and forms a prominent 

 swelling on the under surface of the embryo, between the head 

 and the yolk-sac. It is a single tube, of considerable size ; 

 attached, both in front and behind, to the floor of the fore-gut r 

 but free along the middle portion of its length, which is twisted 

 into a prominent S-shaped loop. The dorsal and posterior end 

 of the loop is the auricular portion of the heart ; and is separated 

 by a slight constriction, the canalis auricularis, from the succeed- 

 ing or ventricular portion (Fig. 232, RV). This forms the widest 

 and most prominent part of the loop ; it runs almost transversely 

 across the body from the left to the right side, and then turns, 

 forwards rather sharply, and passes into the truncus arteriosus, 

 RT, or terminal limb of the loop. The anterior end of the 

 truncus arteriosus (Fig. 232) is attached to the floor of the 

 fore-gut, very far forwards, opposite the mandibular arches. 



The wall of the heart (Fig. 232) is double along its entire 

 length, consisting of an outer mesoblastic tube, in which muscle- 

 cells are already present on the fifteenth day, and an inner 

 endothelial tube, the origin of which has not been determined. 

 The endothelial tube is very much smaller than the muscular 

 tube, and the space between the two is occupied by a gelatinous 

 substance traversed by fine radial fibres, apparently of the 

 nature of connective tissue (cf. Fig. 215). 



During the third week the heart continues to increase rapidly 

 in size, and its several divisions become more clearly marked off 

 from one another by constrictions. By the end of the third 

 week it has reached the condition shown in Figs. 198 and 215. 

 The auricular portion (Fig. 198, RA) is much larger than before ; 

 it is very wide from side to side, and is produced into conspicuous 



