OF ARTS AND SCIENCES. 127 



nal meatus. Whereas its termination maintains the same, or nearly 

 the same, relation to the pharyngeal cavity as before, it has approached 

 much nearer to the inner portion of the cleft, inasmuch as the thicken- 

 ing of the walls of the head in this region has slightly straightened the 

 latter, and brought its long diameter into a position more nearly per- 

 pendicular to the surface of the head. At the same time there has 

 been a reduction in the thickness of the partition (primitive tympanic 

 membrane) which separates the inner from the outer remnant of the 

 cleft. This partition subsequently becomes still thinner (Fig. 20), but 

 according to Kolliker and others it retains a considerable thickness 

 during the whole of embryonic life. 



To recapitulate, the external meatus is formed by the outward 

 growth of the tissue constituting the walls of the first visceral cleft. 

 The closure of the cleft is effected from its ventral edge toward its 

 dorsal extremity by the gradual upgrowth of tissue, which in turn re- 

 sults from the fact that the fusion of the contiguous arches progresses 

 dorsally. The position of the dorsal extremity, which is the last to 

 close, is for some time marked by an external depression (a), but this 

 afterwards becomes more shallow, whereas the ventral portion, which 

 soon after the closure of the dorsal end of the cleft is indicated by only 

 a shallow depression, gradually increases in depth, and ultimately forms 

 the deep portion of the external meatus. 



The tympanic membrane originates from tissue in the region where 

 the cleft is first closed ; it is not uniform in thickness at first, and it 

 lies inclined to the surface of the embryo ; afterwards its thickness is 

 reduced, and at the same time becomes more uniform. Simultaneously 

 the plane of the tympanic membrane is shifted, from a position approx- 

 imately parallel to the surface of the head, to one more nearly perpen- 

 dicular to that surface. The inner remnant of the cleft increases 

 slightly in length, but probably not much, until the embryo is 18 mm. 

 long ; this, moreover, is not due to an actual evagination, but appears 

 to be accomplished by the growth of the tissue forming its walls. Its 

 blind end enlarges to form the tympanum, while the remaining less 

 altered part forms the Eustachian tube. 



The first cleft is closed at a much earlier stas:e than observers have 

 hitherto claimed. The hyoid and mandibular walls come in contact 

 along a line parallel to the surface of the embryo and a little nearer 

 the external than the internal opening of the cleft. From the contigu- 

 ous walls are proliferated cells which form the bridge across the cleft. 

 Simultaneous with this process occurs the formation of the incipient 

 auricle, the changes in the deeper portion of which soon separate the 



