THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 623 



There early grow out from the two superior maxillaries into the 

 palatal processes horizontal lamellae which produce the two palatal 

 bones the hard or bony palate. 



Palatals and pterygoids are developed in the roof and side walls of 

 the oral cavity ; they are consequently mucous-membrane bones. 

 The pterygoids apply themselves, as was stated on p. 620, to the 

 cartilaginous downgrowths of the greater wings of the sphenoid. 

 In many Mammals they remain separate from the latter throughout 

 life, but in Man they unite with it and are now distinguished as 

 inner pterygoid plates from the outer plates, which arise by ossifica- 

 tion of cartilage. 



The development of the visceral skeleton, which has been discussed 

 here and in previous sections (pp. 284, 515), furnishes the basis for 

 the interpretation of the malformations which are quite frequently 

 met with in the maxillary and palatal region in Man. I refer to the 

 labial, maxillary, and palatal fissures, which are simply malformations 

 due to arrested development. They result when the separate funda- 

 ments from which are formed the upper lip, the upper jaw, and the 

 palate do not come into normal union (figs. 288-91). 



The malformations of arrested development can present very 

 different variations, according as the coalescence is wholly or only 

 partly omitted, and according to whether it affects one or both 

 sides of the face. 



In the case of total arrest, in palatal, maxillary, and labial fissures 

 of both sides, both nasal cavities are broadly in communication with 

 the oral cavity by means of a right and a left fissure running from 

 in front backward. From above there projects free into the oral 

 cavity the nasal septum, which is enlarged in front, and here bears- 

 the incompletely developed intermaxillary with its rudimentary 

 incisor teeth. In front of it lies a small dermal ridge, the fundament 

 of the middle part of the upper lip. At the sides of the fissures and 

 the nasal openings, which have not been closed in below, there lie 

 the two separated maxillary processes, with the bony upper jaw and 

 the fundaments of the canine and molar teeth. The horizontal 

 palatal plates project as ridges only a little distance into the oral 

 cavity, and have not effected a junction with the nasal septum. A 

 malformation of this kind is very instructive for the comprehension 

 of the normal processes of development previously described. 



When the arrest is only partial, coalescence may fail either on the- 



