624 EMBRYOLOGY. 



superior maxillary processes only, or on the palatal plates only, and 

 either on one or on both sides. In the first case there is produced a 

 labio-maxillary fasure, or even a labial fissure (hare-lip) only, while 

 hard and soft palates are formed normally. In the other case the 

 upper jaw is well developed and no external evidence of malforma- 

 tion is visible, while there is a fissure on one or both sides which 

 passes through the soft palate, and sometimes through the hard 

 palate also (cleft palate). 



The development of the lower jaw is coupled with fundamental 

 metamorphoses. As has been previously explained, in the youngest 

 embryos the oral cavity is limited below by the membranous inferior 

 maxillary processes. Within this there is developed (fig. 338) 

 MECKEL'S cartilage (MK\ the cranial end of which becomes (compare 

 p. 611) the fundament of the malleus (ha), by means of which 

 MECKEL'S cartilage is articulated with the incus (am). At its 

 ventral end in Mammals it unites in the middle line with the 

 corresponding part of the other side, whereas in Man a small space 

 remains between them. 



Inasmuch as the small cartilages mentioned have arisen in the 

 first visceral arch, they correspond both in position, and also in their 

 mutual connections and many other relations, to the large carti- 

 laginous elements with which we have already become familiar in 

 the Selachians (fig. 330) as palato-quadratum (0) and mandibulare 

 (U). In the Selachians the palato-quadratum and mandibulare are 

 functional as a genuine jaw-apparatus, for they bear on their 

 margins the teeth, which are attached in the mucous membrane 

 only, and the masticatory muscles are inserted on their surface. 



In Mammals and Man the function of the skeletal parts corre- 

 sponding to them has become essentially different, for they have 

 entered into the service of the auditory apparatus ; a profound, :ind 

 in its final results wonderful and highly important metamorphosis 

 has taken place here. In order to explain this it is necessary to 

 touch briefly upon a few comparative-anatomical facts. 



With the beginning of ossifications the primary lower jaw oses in 

 Teleosts, Amphibia, and Reptiles its simple condition, and is con- 

 verted into an apparatus which is often very complicated. The 

 ossifications are here, just as was the case in the other parts of the 

 head-skeleton, of two different kinds, primary and secondary. One 



