The Development of the Frog 75 



distinct, and probably of dermal origin — the membrane 

 bones — graft themselves to it. 



" The three morphologically distinct elements of which 

 the skull consists may with advantage be described sepa- 

 rately. 



" a. The cranium, or brain case. — This in its fully 

 formed condition is an unsegmented cartilaginous tube, 

 enclosing the brain: it is developed as follows : 



" In the front part of the head a pair of longitudinal 

 cartilaginous bars, the trabecula cranii, appear in tad- 

 poles of about lo mm. length: these grow back along- 

 side of the notochord as a pair of horizontal parachordal 

 rods (Fig. 32). 



" 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 trabecular 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 in- 

 wards 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 trabeculae, forms a firm car- 

 tilaginous floor to the brain case. At their hinder ends 

 the parachordals grow upwards to form the side walls 

 of the cranium. Further forwards the pituitary foramen 

 becomes closed by a thin plate of cartilage, and the lat- 

 eral margins of the parachordals and trabeculae grow 

 upwards so as to form the side walls of the skull, the 

 roof remaining imperfect in this region. 



"The first bone to be developed is ihs parasphenoid. 



