984 DEVELOPMENT OF THE OSSEOUS SYSTEM. 



rudiments of the ribs become later the processus costarii (transversi of earlier 

 writers). Occasionally a thirteenth rib is formed. The accessory process of 

 the lumbar vertebne is the true transverse process, as is easily demonstrable in 

 the skeleton of the ape. The sacral vertebras have likewise 3 or 4 rudimentary 

 ribs which after the sixth year unite with the superficies auricularis. The rib- 

 piece has not yet been found on the coccygeal vertebrae. 



The cranium, the closed extremity of the vertebral canal, contains the chorda 

 in the axial part of its base up to the anterior sphenoid body. It is at first entirely 

 membranous (membranous primordial cranium) ; then the basal portions become 

 cartilaginous in the second month, being held together as if cast from a mold: the 

 occipital bone, with the exception of the upper half, the anterior and posterior 

 sphenoids with the wings, the petrous and mastoid portions of the temporal 

 bone, the ethmoid with the nasal septum, and the imperfectly developed external 

 cartilaginous portion of the nose. The other portions of the cranium remain 

 membranous. Accordingly, a membranous and a cartilaginous primoidal cranium 

 have been distinguished. In animals (pigs) the entire occipital and a portion 

 of the parietal region may become cartilaginous. 



Ossification of the individual bones of the skull is completed as follows: 



I. The occipital bone receives in the third month a center of ossification in 

 the basilar portion, one each in the condyloid portion and in the fossa for the 

 cerebellum. In addition two centers occur in the membranous fossae for the 

 cerebrum. The four centers of the bone unite during intrauterine life, although 

 a cleft can be seen on each side from the border between the upper and the lower 

 portion of the squamous portion. Between the first and the second year all the 

 other points unite. Rarely the upper half of the squamous portion persists, as 

 the analogue of the interparietal bone, which is constant in many animals; this is 

 an independent semilunar-shaped bone (of which the author possessed a beautiful 

 example); occasionally one-half of this portion. It should be pointed out as 

 particularly important (also with reference to the development of the brain) 

 that in man the upper portion of the occipital bone enlarges in the process of 

 development, while in apes, on the contrary, it diminishes in size. In some 

 skulls the upper and lower halves of the occipital bone exhibit differences in 

 development. According to Albrecht, the anterior part of the basilar portion 

 forms a special piece of bone, the basioticum. 



II. The postsphenoid has the following centers of ossification from the third 

 month on: two in the sella turcica; two in the carotid groove, and two in both 

 great wings, which form also the external plate of the pterygoid process (while 

 the previously formed noncartilaginous internal plate is derived from the superior 

 maxillary process of the first visceral arch). In the second half of fetal life, these 

 centers unite to from the great wings. The dorsum sellae and the clivus remain 

 cartilaginous up to the spheno-occipital synchondrosis, which ossifies from the 

 thirteenth year on. 



III. The pre sphenoid has from the eighth month two centers in the small 

 wings; then two in the body. In the sixth month these unite, although cartilage 

 is still found within them, remains of which persist until the thirteenth year. 



IV. The ethmoid contains at the fifth month a center in the labyrinth, together 

 with the os planum, the spongy bones and the cribriform plate; then in the first 

 year there is a center in the perpendicular plate and the crista galli. Fusion takes 

 place between the fifth and sixth years. 



V. Among the bones developed from membrane are the inner lamina of the 

 pterygoid process (one center) ; the upper half of the occipital bone (two centers) ; 

 the parietal bone (one center in the parietal eminence) ; the frontal bone (a double 

 center in the frontal eminence) ; in addition three small centers in the nasal spine, 

 the trochelear spine, and the zygomatic process; the nasal bone (one center); 

 the squamous portion of the temporal bone (one center) ; the tympanic ring (one 

 center) ; the lacrimal bone, the vomer and the intermaxillary bones. All of these 

 bones are designated covering or protecting bones; they are formed in a special 

 membranous deposit, which is applied externally to the primordial cranium. 

 O. Hertwig considers them as due to ossification of skin and mucous membrane. 



Croethe appreciated that the cranium of mammals was "derived from ver- 

 tebral bones. The three first vertebrae are admitted: the occipital bone, 

 the postsphenoid, and the presphenoid." The arch of the middle cranial 

 vertebra is closed by the great wings and the parietal bones; the anterior by the 

 frontal bones. The condition is, however, much more complicated. Gegenbaur 

 and btohr, after careful investigation as to the distance the chorda extends an- 



