THE SKELETAL SYSTEM 613 



trally. Later, when ossification begins, short processes called interverte- 

 bral articulatory processes develop from the neural arches, by which 

 each vertebra joins with the next succeeding vertebra. 



Ossification begins in the tadpole between the dorsal and ventral 

 series of cartilages just described. There are nine vertebrae formed in 

 the frog, plus the urostyle, the latter being unsegmented. 



771 



Fig. 352. 



Cross section through a. developing vertebra, rib and 

 exoskeleton of a Turtle, c, corium in which the dermal 

 plates are developed ; cs, primitive vertebral body ; ep, 

 epidermis ; m, external oblique muscle ; p, perichondrium ; r, 

 rib; sp, spinous process. (From Kingsley after Gotte. ) 



THE SKULL 



The skull is commonly formed from various embryological elements, 

 which may be listed as follows : 



(1) Cranium. 



(2) Sense Capsules. 



(3) Visceral Arches. 



(4) Notochord. 



(5) Vertebrae. 



(6) Membrane or Derm Bones. 



It will be remembered that there are no true segments in the head 

 region of the frog. Consequently, the list just given is only assumed 

 from a comparison of other forms. 



When the cranial region begins its cartilage formation, the tadpole 

 Is about seven millimeters in length. A pair of dense strands of tissue 

 form along the ventro-lateral surfaces of the fore-brain. These then 

 become cartilaginous and form the beginnings of the trabeculae, or 

 trabecular cartilages (Fig. 310). These trabeculae extend forward and 

 fuse across the midline between the olfactory organs. The fusion forms 

 the internasal plate. The trabeculae continue extending forward, and 

 these extensions are known as the trabecular cornua, at the ends of 

 which the olfactory capsules form. 



A pair of labial or suprarostral cartilages which have formed in the 

 upper lip meet with the olfactory capsules. 



The notochord which extends into the brain up to the pituitary body 

 has a pair of tissue thickenings beginning in the region of the hind-brain 



