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EMBRYOLOGY. 



the fissures they are no longer recognisable as distinct parts, unless 

 perhaps the thyi-oid cartilage is to be referred to them (Dubois). 



I will describe the conditions in detail, first in the case of sheep 

 embryos of different stages of development, and then in the case of 

 a human embryo. 



In a sheep embryo 2 cm, long there are to be found, according 



to the account of 

 Salensky (fig. 333), 

 two long and slender 

 cylindrical cartila- 

 ginous rods, one in 

 front, the other be- 

 hind the first visceral 

 cleft ; their posterior 

 (proximal) ends abut 

 upon the labyrinth- 

 region of the primor- 

 dial cranium, and are 

 here united to each 

 other by means of 

 embryonic connective 

 tissue. In older em- 

 bryos (fig. 334) the 

 first visceral arch be- 

 comes at its upper 

 [proximal] end more 

 and more distinctly 

 segmented, by means 

 of constrictions, into 

 two smaller pieces 

 and a larger one. 

 The first small piece, 

 the one lying next 

 to the wall of the 

 labyrinth, gradually assumes the form of the incus {am) with its 

 processes, the second becomes the malleus {ha) ; the two are joined 

 by means of a mass of connective tissue. The third piece (mk) is of 

 considerable length, and has the form of a cylindrical rod; it is 

 enclosed in the membranous lower jaw, and is designated in honor 

 of its discoverer as Meckel's cartilage. It remains for a long time 

 in union with the fundament of the malleus by means of a narrow 



Figs. 333, 334. — The dissected-out cartilages of Meckel and 

 Beichebt with the fundament of the auditory ossicles, 

 from a sheep emhryo I3'7 cm. long. After Salensky. 



Fig. 333. — mk, Meckel's cartilage ; Aa, hammer (malleus) ; 

 am, anvil (incus) (long process) ; am', its short process ; 

 z&, cartilaginous hyoid arch. 



Kg. 334. — am, Anvil ; am', its short process ; ha, hammer ; 

 hah, hammer-handle ; st, stirrup (stapes) ; mk, Meckel's 

 cartilage ; zb, cartilaginous hyoid arch. 



