146 DEVELOPMENT OF THE FROG. 



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. 53) 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 the notochord as a 

 pair of horizontal parachordal rods. 



The hinder ends of the trabeculae are some little distance 

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

 lies. In front of this pituitary fossa, the trabeculse 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 

 meet each other both above and below the notochord, which 

 they now completely surround. The two parachordals soon fuse 

 together to form the basilar plate, which, with the trabeculse, 

 forms a firm cartilaginous floor to the brain case. At their 

 hinder ends the parachordals grow upwards to form the side 

 walls of the cranium, and a little later bend inwards so as to 

 meet each other above the brain, and complete the occipital 

 part of the cranium. Further forwards the pituitary foramen 

 becomes closed by a thin plate of cartilage, and the lateral 

 margins of the parachordals and trabeculae grow upwards so as 

 to form the side walls of the skull, the roof remaining im- 

 perfect in this region. 



The first bone to be developed is the parasphenoid. The 

 exoccipitals, the frontals and parietals, which are at first 

 separate, and other bones soon follow ; and by the time the 

 metamorphosis is complete and the tail absorbed, all the bones 

 of the adult cranium are present, except the sphenethmoid, 

 which does not appear till some months later. 



