G20 EMBRYOLOGY. 



is therefore a complex that has originated from five separate 

 bones. 



(2) The sphenoidale also arises from numerous centres of ossifica- 

 tion, which appear in the base of the primordial cranium, and which 

 in the lower classes of Vertebrates represent parts of the cranial 

 capsule that remain separate. In the anterior prolongation of the 

 pars basilaris of the occipitale there appear in the vicinity of the 

 sella turcica an anterior and a posterior pair of centres, which con- 

 stitute the fundaments of the bodies of the anterior and posterior 

 sphenoidea. At the sides of these there are developed special centres 

 of ossification for the lesser and for the greater wings. 



In most Mammals the lesser wings fuse with the anterior, the 

 greater with the posterior body. Thus there are formed two 

 sphenoidea, an anterior and a posterior, which are placed in front of 

 the occipitale, and are separated from each other by a thin strip of 

 cartilage. In Man these two bones become joined together, by the 

 ossification of the cartilaginous strip mentioned, to constitute the 

 unpaired single sphenoidale, with its many processes. The fusions 

 of the numerous separate ossifications take place in the following 

 order. In the sixth foetal month the lesser wings of the sphenoid 

 fuse with the anterior body ; shortly before birth the latter unites 

 with the posterior body, and in the first year after birth the greater 

 wings are united with the rest. From the latter the outer pterygoid 

 plates grow downward, whereas the inner ptery //aid plates are formed 

 as covering bones. For in the connective tissue of the lateral wall of 

 the oral cavity there is developed a special region of ossification ; 

 this furnishes a thin bony lamella, which is preserved in many 

 Mammals as a special skeletal element (os pterygoideurn) lying on 

 the pterygoid process of the sphenoidale. In Man it early fuses 

 with the sphenoidale, notwithstanding it has an entirely different 

 origin from the latter. 



(3) The temporale is a complex of various bones, the greater part of 

 which are still separate in the new-born infant. The os petrosum 

 with the mastoid process is developed from numerous centres of 

 ossification in that part of the primordial cranium which encloses the 

 organ of hearing, and has therefore been designated as cartilaginous 

 ear-capsule. With it is united after birth the styloid process, which 

 in the embryo is a cartilaginous rod that is derived from the upper 

 end of the second visceral arch and that ossifies from its own 

 independent centre. 



To the primordial bones there are added in Man two covering 



