140 DEVELOPMENT OF THE FROG 



vertebra, ossification gradually spreads inwards ; but a small 

 portion of notochord persists in the middle of each centrum 

 for a long time, or even throughout life. 



The vertebrae are not placed opposite the myotomes, but 

 alternate with these ; so that each vertebra is acted on by two 

 myotomes on each side, one pulling it forwards, and the other 

 backwards. 



The transverse processes are at first independent of the 

 col-responding vertebrae, but very early fuse with them. They 

 extend into the septa between the myotomes, and probably 

 correspond to the ribs of other vertebrates. 



The urostyle is the part of the axial skeleton behind the 

 vertebne ; it is not divided into vertebrae at any stage in 

 development. 



The anterior end of the notochord, imbedded in the base of 

 the skull, is gradually encroached on by the cartilage and bone 

 around it, and ultimately completely absorbed. 



2. The Skull. 



The skull of the tadpole consists almost entirely of cartilage ; 

 none of the bones of the skull, with the exception of the para- 

 sphenoid, appearing until nearly the time of the metamorphosis. 

 In the adult frog, this cartilaginous skull is replaced to a con- 

 siderable extent by cartilage-bone; while other bones primitively 

 distinct, and probably of dermal origin the membrane-bones 

 graft themselves on to it. 



The three morphologically distinct elements of which the skull 

 consists (cf. p. 43) may with advantage be described separately. 



a. The Cranium or brain case. This in its fully formed con- 

 dition is an unsegmented cartilaginous tube, enclosing the brain 

 it is developed as follows : 



In the front part of the head a pair of longitudinal cartilagi- 

 nous bars, the trabeculse cranii, appear in tadpoles of about 10 

 mm. length : these grow back alongside of the notochord as a 

 pair of horizontal parachordal rods. 



The hinder ends of the trabeculse are some little distance 

 apart, and between them is a space in which the pituitary body 

 lies. In front of this pituitary fossa, the trabeculre unite to 

 form a plate of cartilage, which underlies the anterior end of the 

 brain, and is produced into blunt processes at its outer angles. 



The parachordals grow rapidly : they extend inwards so as to 



