88 THE SCAFFOLDING LEFT IN THE BODY. 



mainly effected by a series of remarkable develop- 

 ments of one of the now superfluous gill-slits. 



It has long been a growing certainty to Comparative 

 Anatomy that the external and middle ear in Man are 

 simply a development, an improved edition, of the 

 first gill- cleft and its surrounding parts. The tym- 

 pano-Eustachian passage is the homologue or counter- 

 part of the spiracle associated in the shark with the 

 first gill-opening. Prof. His of Leipsic has worked 

 out the whole development in minute detail, and con- 

 clusively demonstrated the mode of origin of the 

 external ear from the coalescence of six rounded 

 tubercles surrounding the first branchial cleft at an 

 early period of embryonic life.^ 



1 Haeckel lias given an earlier account of the process in the 

 following words : — " All the essential parts of the middle ear — the 

 tympanic membrane, tympanic cavity, and Eustachian tube — 

 develop from the first gill-opening with its surrounding parts, 

 which in the Primitive Fishes (Selachii) remains throughout life 

 as an open blow-hole, situated between the first and second gill- 

 arches. In the embryos of higher Vertebrates it closes in the 

 centre, the point of concrescence forming the tympanic mem- 

 brane. The remaining outer part of the first gill-opening is the 

 rudiment of the outer ear-canal. From the inner part originates 

 the tympanic cavity, and further inward, the Eustachian tube. 

 In connection with these, the three bonelets of the ear develop 

 from the first two gill-arches ; the hammer and anvil from the 

 first, and the stirrup from the upper end of the second gill-arch. 

 Finally, as regards the external ear, the ear-shell (concha auris), 

 and the outer ear canal, leading from the shell to the tymi3anic 

 membrane — these parts develop in the simplest way from the skin 

 covering which borders the outer orifice of the first gill-opening. 

 At this point the ear-shell rises in the form of a circular fold of 

 skin, in which cartilage and muscles afterwards form." — Haeckel, 

 Evolution of Man, Vol. ii., p. 260. 



