THE SKULL. 203 



pacted, so that the nuclei of adjacent cells lie very close to one 

 another, while the protoplasm of the several cells becomes fused 

 into a continuous plasma or matrix. Very slight further changes 

 convert this into true cartilage. 



(i) The cranium or brain case. This, in its fully formed 

 condition (Fig. 93), is an unsegmented cartilaginous tube, in- 

 complete dorsally, and inclosing the brain. It is developed in 

 the following manner. 



Shortly after hatching of the tadpole, a pair of longitudinal 

 p< ils of condensed mesoblast, the trabecule cranii, appear, one 

 on either side of the anterior end of the brain. These are at 

 first entirely in front of the notochord (cf. Fig. 64) : their pos- 

 terior ends lie alongside the fore-brain ; while anteriorly they 

 are flattened dorso-ventrally, and fused to form a horizontal 

 plate lying between the two nasal sacs ; the extreme anterior 

 ends of the trabecule) separate again, and bend downwards into 

 the upper lip, which they support. 



As the tadpole grows, the trabecular rapidly extend back- 

 wards along the sides of the infundibulum, and soon reach the 

 anterior end of the notochord ; they continue backwards along- 

 side the notochord, and in close contact with this, as a pair of 

 horizontal rods, the parachordals, which with the notochord form 

 the floor of the brain case. 



In tadpoles of 12 mm. length (Figs. 90, 91, and 92) the 

 trabecules have become cartilaginous ; their extreme anterior 

 ends chondrify independently as a pair of thin curved plates, 

 the upper labial cartilages, LU, which support the upper lip. 

 Behind the labial cartilages the two trabecule remain separate 

 for a short distance, but soon fuse to form a median horizontal 

 plate of cartilage, lying between the two nasal sacs, and sup- 

 porting the extreme anterior end of the brain ; behind this they 

 are continued backwards as two parallel bars of cartilage, lying 

 at the sides of, and slightly ventral to the brain ; behind the 

 infundibulum, and just in front of the anterior end of the noto- 

 chord (Fig. 91), the two trabecule? are again connected by a 

 narrow transverse bridge of cartilage, beyond which they continue 

 backwards, as two narrow parachordal bands, RC, along the sides 

 of the notochord, to the hinder end of the skull. 



In the later tadpole stages the trabecules extend horizontally 



