THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 613 



cartilaginous bridge, upon which the long process (pr. gracilis) of 

 the malleus is afterwards developed by periosteal ossification. The 

 second visceral arch (zb) becomes incorporated in the hyoid bone. 



In a human embryo of the fifth month one observes structures 

 similar to those just described, only somewhat further developed 

 Figure 335 exhibits the incus (am), easily recognised by its form, 

 lying on the wall of the labyrinth ; with it is articulated the malleus 

 (ha), the long process of which is continuous with MECKEL'S cartilage 

 (MK). This extends ventrally as far as the median line, where it 

 is united with the cartilage of the opposite side by means of con- 

 nective tissue a kind of symphysis. 



The second visceral cartilage, called also KEICHERT'S cartilage, has 

 become divided into three portions. The uppermost portion is fused 

 with the labyrinth-region the petrous portion of the temporal bone 

 and constitutes the fundament of the processus styloideus (grf) 

 the middle portion has become fibrous tissue in Man, and forms 

 n strong ligament, the lig. stylohyoideum (Isth), whereas in many 

 Mammals it becomes a large cartilage ; the third and lowest portion 

 produces the lesser cornu (kh) of the hyoid bone. This sometimes 

 becomes developed to a great length by the chondrification of the 

 lower part of the ligamentum stylohyoideum, and reaches up very 

 close to the lower end of the stylohyoid process. 



In the third visceral arch chondrification takes place only in the 

 ventral tracts, producing upon the sides of the neck the greater cornua 

 of the hyoid bone (gh). Greater and lesser cornua are attached to 

 an unpaired median piece of cartilage, which corresponds to a copula 

 of the visceral skeleton of Selachians and becomes the body of the 

 hyoid bone. 



The third auditory ossicle, the stapes (fig. 335 si), also belongs to 

 the visceral apparatus ; it has been left unmentioned until now, 

 because there is, even at present, a wide difference of opinion con- 

 cerning its development. According to the original view of REICHERT, 

 which GEGENBAUR is also inclined to adopt, the stapes arises from 

 the uppermost end of the hyoid arch. KOLLIKER refers it to the 

 first visceral arch. According to GRUBER and PARKER, on the 

 contrary, it arises in connection with the fenestra ovalis, as though 

 it were cut directly out of the outer wall of the labyrinth. 



According to the recent investigations of SALENSKY, GRADENIGO, 

 and KABL, it appears to me that the, stapes has a double origin, 

 arising from two different parts. 



The plate of the stapes, which is let into the fenestra ovalis, is 



