THE VISCERAL ARCHES AND CLEFTS. 285 



The hyomandibular cleft opens to the exterior in the latter 

 part of the third day (Figs. 113 and 124, HM) ; it remains open 

 until about the end of the fourth day, when its walls come in 

 contact, and the cleft becomes closed. The first branchial cleft, 

 between the hyoid and first branchial arches, opens a little 

 later, early on the fourth day ; and closes again during the fifth 

 day. The second branchial cleft, between the first and second 

 branchial arches, is open only for a short time during the fifth 

 day : and the third branchial cleft does not open to the exterior 

 at any time. 



It is stated by some observers that none of the visceral clefts 

 in the chick open to the exterior at any stage, but the real con- 

 dition appears to be as described above ; it is possible that indi- 

 vidual variations occur in respect to the dates of opening of the 

 clefts, and the times during which they remain open. 



The tympano-Eustachian passage. The branchial clefts close 

 up and disappear completely at an early stage ; but the most 

 anterior, or hyomandibular, cleft appears to persist, and to give 

 rise directly to the tympano-Eustachian passage of the adult bird. 

 The cleft becomes closed at its outer end, about the end of the 

 fourth day, by a fold of skin, which becomes directly the tympanic 

 membrane. From the gill-pouch, on the inner side of the tym- 

 panic membrane, the tympanic cavity and Eustachian passage 

 are formed ; while the external auditory meatus is built up as a 

 short tubular passage on the outer side of the tympanic mem- 

 brane (Fig. 126, HM). The Eustachian passages of the two sides 

 unite at their inner ends, and open into the mouth by a median 

 aperture (Fig. 116, ES), nearly opposite the glottis. 



According to some observers, the hyomandibular pouch does 

 not open to the exterior at any period in the chick ; and the 

 tympanic membrane is formed directly from the thin double 

 layer, consisting of both epiblast and hypoblast, which closes the 

 pouch at its outer end ; a layer of mesoblast growing in between 

 the epiblast and hypoblast, which persist as the epithelial 

 layers of the outer and inner surfaces of the tympanic membrane 

 respectively. The whole history of the development of these parts 

 stands in need of renewed and thorough investigation. 



The thyroid body arises, towards the end of the second day, 

 as a median longitudinal groove in the floor of the pharynx. 



