158 



TEXT-BOOK OF EMBRYOLOGY. 



of the periotic capsule. The cartilages of the other three arches are only 

 indirectly connected with the skull and will be considered later. 



Figs. 135 and 136 show the condition of the chondrocranium in a human 

 embryo of 80 mm. (third month) . Although at first glance it seems exceedingly 

 complicated, a careful study and comparison of the various parts will aid the 

 student in his comprehension of the cartilaginous foundation upon which the 

 skull is built. 



OSSIFICATION OF THE CHONDROCRANIUM. 



In the human fcetus ossification begins in the occipital region during the 

 third month. Four centers appear which correspond to the four parts of the 

 adult occipital bone (Fig. 137). (i) An unpaired center situated ventral to the 

 foramen magnum. From this center ossification proceeds in all directions to 



Interparietal 

 (of lower forms) 



Squamous part 

 (intramemb.) 



Squamous 

 part 



Kerkringius' bone 



Squamous part 

 '(intracartilag.) 



Lateral part 



Basilar part 



FlG. 137. Occipital bone of human embryo of 21.5 cm. Kollmann's Atlas. 



form the pars basilaris (basioccipital). (2 and 3) Two lateral centers, one 

 on each side. From these, ossification proceeds to produce the partes laterales 

 (exoccipital) which bear the condyles. (4) A center dorsal to the foramen 

 magnum. This produces the pars squamosa (supraoccipital) as far as the supe- 

 rior nuchal line. Beyond this line the pars squamosa is of intramembranous 

 origin. (See p. 160.) At birth the four parts are still separated by plates of 

 cartilage. During the first or second year after birth the partes laterales 

 unite with the pars squamosa, and about the seventh year the pars basilaris 

 unites with the rest of the bone. 



