94 INTRODUCTION. 



tadpole condition are the exoccipitals and prootics 

 (cartilage bones) and the fronto-parietals, or frontals 

 and parietals if these ossify from distinct centres, and 

 parasphenoid (membrane bones). The prsemaxillaries, 

 maxillaries, and squamosals, next in order, appear only 

 some time after the loss of the suctorial or larval 

 mouth. 



Whilst the fore limbs are enclosed, each moiety of 

 the shoulder-girdle is widely separated from the other ; 

 the scapula is directed upwards, and the coracoid and 

 prEecoracoid form a loop turned downwards and 

 inwards. At the same period the ilium is per- 

 pendicular to the vertebral column. 



The tail proper of the tadpole remains in a noto- 

 chordal condition, no cartilage being ever formed in 

 that region. But both dorsal and ventral cartilages 

 are developed in the basal portion of the caudal region, 

 and ossify as two dorsal elements or arches and a 

 single ventral strip behind the ninth or sacral verte"bra ; 

 they gradually fuse to a continuous cylinder, the 

 urostyle or coccyx, from which both chorda and spinal 

 cord ultimately disappear, to form in the adult a solid 

 bony rod. 



The vertebras of the trunk are formed, as first 

 discovered by Dnges, on two different plans. In the 

 Discoglossidm, Pelobatidae, and Hylidse the chorda 

 remains for a long time exposed along the ventral 

 surface, and, owing to the absence of cartilaginous 

 formation around it, disappears without ever becoming 

 invested otherwise than by a thin elastic membrane ; 

 it can easily be stripped off below the vertebras in 

 specimens on the point of metamorphosing. This has 

 been termed by Gegenbaur the epichordal type. In 

 the Biifonidse and BanidsB, which represent the peri- 

 ehordal type, the greater share of the formation of the 

 whole vertebra falls to the (paired) dorsal cartilage, 

 but there is in addition a narrow ventral or hypo- 

 chordal cartilage which fuses with the dorsal or 

 becomes connected with it by calcified tissue ; the 



