THE SKULL. 203 



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

 another, while the pi'otoplasm 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 cas e. This, in its fully formed 

 co ndition (Fig. 93), i s an unsegmented cartilaginous tube, in- 

 co mplete dorsally, an d inclosing the brain . It is developed in 



the fnHnwing- mnnnAv 



Shortly after hatching of the tadpole, a pair of longitudin al 

 r ods of condensed mesoblast. t he t rabeculse cranii, appear, one 

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

 first entirely in front of the notochord (c/. 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 trabeculas separate again, and bend downwards into 

 the upper lip, which they support. 



As the tadpole grows, the trabeculas 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 nai'achordals. whic h with the notochord form 

 the fl oor of the brain ci^ se. 



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

 trabeculse have become cartilaginous ; th gir extre me a nterior 

 ends^ ch ondrify i nd ependently as a pair of thin curved plates. _ 

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

 Behind the labial cartilages the two trabeculse 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 thej' 

 are continued backwards as two parallel bars of cartilage, lying 

 at the sides of, and slightlj- ventral to the braiu ; behind the 

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

 chord (Fig. 91), the two trabeculse 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 trabeculse extend horizontally 



