300 THE CHICK. 



straight tube, lying below the fore-gut, and closely attached to 

 its ventral surface, and with its walls consisting of an outer 

 muscular tube and an inner endothelial tube. 



The posterior end of the heart is, from the first, continuous 

 with, or rather is formed by the union of, two large vessels, the 

 vitelline veins (Fig. 127, vv), which collect and return to the 

 embryo the blood from the network of capillaries in the area 

 pellucida and area opaca (cf. Fig. 98). 



The heart is at first very short ; but as the head fold becomes 

 deeper, constricting the embryo more and more markedly from 

 the yolk-sac, the vitelline veins remain at the edge of the fold, 

 and so get carried back with it, causing thereby lengthening of 

 the heart. 



The heart begins to beat very shortly after its first formation, 

 and before any distinct histological differentiation into muscle 

 and nerve cells can be distinguished. 



On its first formation, the heart is attached along its whole 

 length to the under surface of the fore-gut. It remains attached 

 at its two ends, but about the thirty-third hour becomes free 

 along the middle portion of its length (Fig. 112, RV) ; and, 

 growing more rapidly than the parts to which it is attached, 

 becomes thrown into a loop (Fig. 127), with the convexity 

 towards the right, and the concavity towards the left side of 

 the embryo. 



The loop, continuing to lengthen, projects downwards and 

 backwards, so that the whole heart, towards the end of the second 

 day, becomes twisted obliquely, into a letter S shape. Starting 

 from the point of union of the vitelline veins, the heart runs for- 

 wards a certain distance, then makes a sharp bend downwards, 

 backwards, and to the right side ; then, making a second, equally 

 sharp bend (cf. Fig. 113, RV) upwards, forwards, and to the left, 

 reaches the median plane again, and is attached to the under 

 surface of the pharynx opposite the first two pairs of gill-clefts. 



The posterior end of the heart, into which the vitelline veins 

 open, may be called the venous end of the heart ; and the 

 anterior end, the arterial end. The first bend or loop (Fig. 

 113, RA) marks the auricular part of the heart ; and the second 

 bend, RV, the ventricular part. The forwardly directed portion 

 of the heart, in front of the second bend, is the truncus arteri- 

 osus, RT. 



