614 



EMBRYOLOGY. 



differentiated in the manner first emphasised by GRUBER and PARKER, 

 and now again by GRADENIGO, out of the cartilaginous capsule of the 

 labyrinth. Its development therefore agrees with that of the oper- 

 culum of the Amphibia, as described by STOHR. The ring-like part 

 of the stapes, on the contrary, comes from the upper end of the 

 second visceral [hyoid] arch, which lies in contact with the capsule 

 of the labyrinth (GRADENIGO, EABL). Its ring-like condition results 



ha 



MK 



grj isth. gh 



Fig. 335. Head and neck of a human embryo 18 weeks old with the visceral skeleton exposed, 



after KOLI/.KER. Magnified. 

 The lower jaw is somewhat depressed in order to show MECKEL'S cartilage, which extends to the 



malleus. The tympanic membrane is removed and the annulus tympanicus is visible. 

 ha, Malleus, which passes uninterruptedly into MECKEL'S cartilage, MK ; v.k, bony lower jaw 



(dentale), with its condyloid process articulating with the temporal bone ; am, incus ; 



st, stapes ; pr, annulus tympanicus ; grf, processus styloideus ; Isth, ligamentum stylo- 



hyoideum ; kh, lesser cornu of the hyoid bone ; gh, its greater cornu. 



from the fact that the tissue from which it is formed is traversed 

 by a small branch of the carotis interna, the arteria mandibularis 01 

 perforans stapedia. In Man and certain of the Mammals this 

 subsequently degenerates entirely, whereas in others (Rodents, In- 

 sectivores, etc.) it remains as a vessel of considerable size. 



Both fundaments of the stapes fuse with each other very early 

 and form a small cartilage, which on the one hand articulates with 

 the incus by means of a lenticular connecting element (os lentiforme), 



