VI.] THE AORTIC ARCHES. 167 



formed of a stalk which is held by Parker to be derived 

 from part of the skeleton of the visceral arches, while 

 the base, forming the stapes, appears to be an inde- 

 pendent formation. 



The stalk of the columella extends to the tympanic 

 membrane; its outer end becoming imbedded in this 

 membrane, and serving to transmit the vibrations of 

 the membrane to the fluid in the internal ear. 



Vascular system. By the end of the second day 

 three pairs of aortic arches had been established in 

 connection with the heart. When the visceral folds 

 and clefts are formed, a definite arrangement between 

 them and the aortic arches is always observed. The 

 first visceral cleft runs between the first and second 

 aortic arches. Consequently the first aortic arch runs 

 in the first visceral fold, and the second in the second. 

 In the same way, the second visceral cleft lies between 

 the second and third aortic arches, the third aortic arch 

 running in the third visceral fold. Each aortic arch runs 

 in the thickened mesoblast of the corresponding fold. 



Arrived at the dorsal surface of the alimentary canal, 

 these arches unite at acute angles to form a common 

 trunk, the dorsal aorta (Fig. 57, A.O), which juus along 

 the back immediately under the notochord. The length 

 of this common single trunk is not great, as it soon 

 divides into two main branches, each of which, after 

 giving off the large vitelline artery, Of. A., pursues its 

 course with diminished calibre to the tail, where it is 

 finally lost in the capillaries of that part. 



The heart is now completely doubled up on itself. 

 Its mode of curvature is apparently somewhat compli- 

 cated. Starting from the point of junction of the vitel- 



