THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYSIE. 621 



hones, squama and pars tympanicus, which are as foreign to the 

 primordial cranium as the parietal or frontal bones. Of these the 

 pars tympanicus (fig. 335 pi') is at first a narrow bony ring, which 

 serves as a frame for the tympanic membrane. It is developed in 

 connective tissue outside of the auditory ossicles, and, in particular, 

 outside the malleus (ha) and the connected MECKEL'S cartilage ( J/7T). 

 Thus is explained the position of the long process of the malleus in 

 the fissura petrotympanica, when, soon after birth, the primordial 

 and covering bones fuse with each other. For the annulus tym- 

 panicus gradually becomes broadened into a bony plate, which serves 

 as a support for the external meatus. This plate then fuses with 

 the petrosal bone, except along a narrow cleft, the fissura petro- 

 tympanica or Glaseri, which remains open, because here the chorda 

 tympani and the long process of the malleus were in the embryo 

 shoved in between the bones, while they were still separate. 



In lower Vertebi-ates, and also in many Mammals, the pieces 

 mentioned remain separate, and are distinguished in comparative 

 anatomy as os petrosum, os tympanicum, and os squamosum. 



(4) The ethmoidale and the turbinatum of the nose are primordial 

 bones, which are developed out of the posterior part of the cartila- 

 ginous nasal capsule, whereas the anterior part remains cartilaginous 

 and becomes the cartilaginous septum nasorum and the external nasal 

 cartilages. 



" The ossification begins in the lamina papyracea in the fifth 

 month. Then follows the ossification of the lower and middle 

 turbinals. At birth these are united by means of cartilaginous 

 portions of the ethmoidale. After birth the vertical plate with the 

 crista galli is the first to ossify; then follows the ossification of the 

 upper turbinal and of the gradually developed labyrinth, from which 

 the ossification advances to the corresponding halves of the cribri- 

 form plate. The union of the two lateral halves with the lamina 

 perpendicularis does not take place until between the fifth and the 

 seventh year." (GEGENBAUR.) 



Of the covering bones of the primordial cranium, which in general 

 begin to ossify at the beginning of the third month, the following 

 remain separate : the parietale, frontale, nasale, lachrymale, and 

 vomer. Of these the frontale is originally, like the others, a paired 

 structure, and still continues in this condition into the second year 

 after birth, when the closure of the frontal suture begins. Nasale 

 and lachrymale are covering bones of the cartilaginous nasal 

 capsule. The vomer arises as a paired structure at the sides of the 



