114 PROCEEDINGS OP THE AMERICAN ACADEMY 



points concerning the external features of the cleft I have been able to 

 confirm his conclusions. Soon after the confluence of the arches on 

 the ventral side, says Reichert, there appear on the margins of the 

 arches which bound the remnant of the first cleft elevations, the apices 

 of which are directed toward each other; each of these elevations lies 

 between two slight depressions. The cleft is thereby made to appear 

 broader at two places (the ends), and narrow in the middle. In the 

 further metamorphosis of the parts there are two points of importance : 

 the changes of these elevations and depressions, and the actual and ap- 

 parent migration of the external opening of the cleft. In stages where 

 the second and third clefts are already closed, it can be ascertained by 

 manipulation that the tissue which seals up the first cleft lies, as Rathke 

 claimed, nearer the external than the internal opening of the cleft. Of 

 the two broadened ends of the cleft, the lower (ventral) becomes the 

 deeper and more important. 



As seen from the side, the arches present, beside the two elevations 

 mentioned, other smaller elevations and depressions, which together 

 with the former surround the visceral opening like two walls. While 

 the lower division of the cleft is continually becoming more evidently 

 metamorphosed into the external meatus, the posterior wall, which is 

 really an outgrowth of the second visceral process, gradually becomes 

 higher, and while its main elevation forms, as it were, the root or stem 

 of the external ear, the rest of it becomes the concha. The anterior 

 wall does not become prominent. 



As regards the second point, the remnant of the cleft at first lies 

 between the upper (dorsal) portions of the first and second visceral 

 processes, and it continues to occupy this position in relation to the 

 forming parts ; the latter, however, are shifted backwards by the for- 

 mation of the lower jaw and the tongue, and in consequence of this 

 the plane of the fissure, which was at first nearly perpendicular to the 

 long axis of the embryo, becomes gradually more oblique. Tlie appar- 

 ent dorsal migration of the external opening, on the other hand, is due 

 to the formation of the facial parts, especially to a gradual broadening 

 of the ventral portion of the embryo in the ear region. 



The inner portion of the cleft is elongated into a canal by the growth 

 of the surrounding tissue. The canal is somewhat narrowed by the 

 encroachment of the labyrinth near the place where the fusion of the 

 walls of the cleft first took place; the portion lying outside the con- 

 striction becomes the tympanum, the rest becomes the Eustachian 

 tube. The latter does not diminish in size, as Valentin claimed, nor is 

 its axis changed in direction, but remains from the beginning directed, 

 from outwards and forwards, inwards and backwards. 



