602 DEVELOPMENT [CHAP.XLTT. 



and at this point a second vesicle makes its appearance, which 

 becomes the cochlea. 



The semicircular canals are developed by a contraction and 

 folding-in of the walls of the vestibule. From Mr. Gray's careful 

 observations, it appears that the labyrinth, about the twelfth or 

 thirteenth day of development in the chick, has an appearance 

 closely resembling the retina at the same time a point of great 

 interest. Huschke has shown that the Eustachian tube, the cavity 

 of the tympanum, and the external meatus, are the remains of 

 the first branchial cleft, which eventually becomes divided by the 

 membrana tympani. 



The ossicles of the ear are formed as follows : The malleus and 

 incus, according to Reichert, are produced from the first visceral 

 arch, which also gives origin to the superior and inferior maxillae. 

 The stapes appears to be produced from the second visceral arch, 

 which also gives rise to the hyoid bone and its suspensory appa- 

 ratus. The ossification of these little ossicles commences in the 

 fourth month of intra-uterine life. 



The development of the mouth and nose have already been 

 alluded to at page 598. 



- Development of the Heart and Aortic Arches. The development 

 of the heart is best studied, in the chick. It appears towards the 

 end of the se'cond day of incubation, as a small hollow tube be- 

 tween the mucous and serous laminae of the germinal membrane. 

 AKout the thirty-sixth hour it has become a simple tube, much 

 curved and twisted upon itself. Posteriorly, it terminates in two 

 or three large venous trunks, which are insensibly lost on the 

 germinal membrane ; and anteriorly, it divides into two branches, 

 which unite beneath the vertebral column to form the aorta. The 

 trunk of the vessel again divides into two branches, which are lost 

 on the vascular area. Early on the third day, the heart consist 

 of three cavities the sinus venosus, the ventricle, and the bulbus 

 aortcB ; the first soon becomes divided into the two auricles, and 

 by the fourth day the ventricle assumes its usual form, and the 

 formation of the septum, which divides its cavity into two portions, 

 commences. 



About the beginning of the third day, the aortic bulb divide 

 into four pair of vascular arches. On the fourth day, the firs 

 pair disappears and is at length obliterated, and the second pail 

 becomes smaller; but now is formed a fifth pair, which becomes 

 larger on the fifth day, while the second entirely disappears ; 

 that there are at this time only three pair, and these of nearly 

 equal size. 



