144 Vertebrate Embryology 



but after the fusion of the tubes the walls are 

 completed. 



The stem of the Y, developed as above 

 described, forms the heart (Fig. 49, C), while 

 the diverging arms of the Y are continuous 

 with the large vitelline veins which bring the 

 blood back to the heart from the vascular 

 area (Fig. 49, VOM). 



At the thirtieth hour, then, the heart is 

 a short, straight tube which is attached to the 

 ventral wall of the fore-gut or pharynx. 



The point of divergence of the vitelline 

 veins is at the hindermost angle of the head- 

 fold, and as the head-fold is pushed farther 

 and farther back, the heart, or straight part 

 of the Y, is correspondingly lengthened. But 

 the tubular heart seems to grow more rapidly 

 than does the place to which it attached, with 

 the result that it is bent into a loop, with the 

 convexity of the loop to the right side of 

 the embryo (Fig. 49, C). This looping of the 

 heart is made possible by the fact that, while 

 it was at first attached to the wall of the fore- 

 gut throughout its whole length, it soon be- 

 comes detached from that wall for the greater 

 part of its length, retaining its attachment 

 only at the ends. The end of the heart into 



