THE EPIMERE (SEGMENTAL OR VERTEBRAL PLATE) 213 



inward growth of true bony cells known as osteoblasts. Eventually the 

 notochord itself becomes invaded by these bone-forming cells and is 

 transformed into the bony centrum of the vertebra. Each centrum 

 becomes concave anteriorly, for the reception of the convex projec- 

 tion of the more anterior centrum. At the level of the lateral myo- 

 tomal mass, connective tissue invades the notochord to form the inter- 

 vertebral discs or ligaments of hyaline cartilage. Each disc splits into 

 an anterior and posterior part, and each becomes ossified and later 

 fuses with the corresponding part of the adjacent centrum. There are 

 connecting ligaments which arise from the original sclerotome along 

 both the dorsal and the ventral faces of the centra. They alternate in 

 position with the myotomal segments. 



The ossified cartilage which surrounds the spinal cord is known as 

 the neural arch. Both the anterior and the posterior margins of each 

 neural arch bear a pair of short zygapophyses which are processes that 

 articulate and tend to join successive vertebrae. Spinal nerves emerge 

 from the spinal cord through intervertebral foramina, between the 

 sides of the succeeding neural arches. 



Dorsal to each potential vertebra surrounding the spinal cord is a 

 single median cone of ossification which becomes the spinous or 

 neural process of the vertebra. The successive neural spines are con- 

 nected by ligaments. The paired transverse and bony processes arise 

 laterally, at right angles, from the cartilage of the centrum and be- 

 come continuous with the minute bony ribs. No transverse processes 

 or anterior zygapophyses are developed on the most anterior vertebra, 

 the atlas. This vertebra is modified to articulate with the occipital 

 condyles of the skull. The transverse processes of the ninth vertebra 

 are elongated and are directed obliquely posterior to provide attach- 

 ment for the ilium of the pelvic girdle. The tenth vertebra is greatly 

 modified into the single tubular urostyle and is derived from the 

 sclerotome (skeletogenous material) of the last two somites of the 

 body. 



The Skull (Neurocranium of the Adult). 



The skull of the adult frog, defined as that part of the skeleton 

 which surrounds and supports the brain and special sense organs, 

 is composed of relatively more cartilage than is the skull of most higher 

 vertebrates. The cranial cartilages are an exception to the general 

 rule as to origin, and are derived from ectodermal neural crests. 



