612 



EMBRYOLOGY. 



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

 perhaps the thyroid 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 (ani) 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 



hah zb 



Fig. 334. 



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

 REICHERT with the fundament of the auditory ossicles, 

 from a sheep eaxbryo 2-7 cm. long. After SALENSKY. 



Fig. 333. -ink, MECKEL'S cartilage ; ha, hammer (malleus) ; 

 aiii, auvil (incus) (long process); am', its short process; 

 zb, cartilaginous hyoid arch. 



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

 hah, hammer-handle ; st, stirrup (stapes) ; mk, MECKEL'S 

 cartilage ; zb, cartilaginous hyoid arch. 



