398 THE EABBIT. 



the external groove, so that the cleft is at this stage closed only 

 by a very thin branchial membrane, EB, formed of epiblastic 

 and hypoblastic layers without any intervening mesoblast. As 

 shown in the figure, the hyomandibular pouch lies at this stage 

 some distance ventral to the ear, and the two structures are 

 completely independent of each other. 



By the fifteenth day (Fig. 159) the conditions have changed 

 materially. A thick layer of mesoblast has grown in between 

 the epiblast and hypoblast of the branchial membrane, so that 

 the hyomandibular pouch is now separated from the surface of 

 the head by a thick plate, EB, which becomes later on the 

 tympanic membrane. Further, by growth upwards of its lips, 

 and through the general thickening of the side walls of the 

 head, the shallow hyomandibular groove of the earlier stage is 

 converted into a deep pit, EO, the external auditory meatus, the 

 margin of which is already commencing to grow out as the 

 rudiment of the pinna or external ear (cf. Figs. 149 and 157). 



In the later stages (cf. Fig. 157), the external meatus, EM, 

 becomes much longer, and the pinna attains enormous dimen- 

 sions ; the tympanic membrane, TM, becomes relatively much 

 thinner than at the fifteenth day ; while the Eustachian passage 

 becomes more distinctly tubular, and, owing to the formation of 

 the palate, P, now opens into the posterior narial chamber 

 instead of directly into the buccal cavity. 



With regard to the auditory ossicles of the rabbit, it is 

 difficult to speak with certainty. The stapes (Fig. 159, SA) 

 forms, about the fifteenth day, as a ring of cartilage, which 

 from its first appearance is in close connection with the outer 

 wall of the periotic capsule, and apparently continuous with 

 this, at the place where the fenestra ovalis is formed a little 

 later. The ring-like form of the stapes is apparently due, as 

 shown in Fig. 159, to its being formed around a small branch 

 of the carotid artery, AC. 



Concerning the origin of the other two auditory ossicles of 

 the mammal, the malleus and incus (Figs. 157, M, and 159, MA), 

 there has been much discussion. While it appears very probable 

 that they are formed in connection with the cartilaginous bars 

 of one or more of the visceral arches, investigators differ widely 

 as to whether both are developed from the mandibular bar, 



