152 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



extend forward from the anterior end of the parachordals (or 

 of the notochord) on either side of the pituitary body. In 

 front, at about the anterior end of the brain, these trabeculas 

 turn inwards towards each other and fuse into a median mass 

 which, from its future history, is known as the ethmoid plate. 

 Farther forward the trabeculae separate, and turn outward in 

 front of the developing olfactory organs, the diverging horns 

 thus formed being known as the cornua trabeculae. The far- 

 ther development of the trabecular region differs considerably in 

 different vertebrates. In general the trabeculae rapidly increase 

 in height by the development of a crest upon the dorsal surface, 



FlG. 160. Chondrocranium of embryo trout {Salmo fontinalis}. h, hyoid ; hy r 

 foramen for hyomandibular nerve ; ix, foramen for glossopharyngeal nerve ; Jv> 

 foramen for branch of jugular vein; ;, Meckel's cartilage; MS, nasal septum; 

 rs, foramen for ophthalmicus superficialis ; sb, supraorbital bar ; st, synotic tectum ; 

 /, trabecula; tc, tegmen cranii. From Winslow. 



and in the elasmobranchs and some ganoids (sturgeon, etc.) this 

 process is continued until the brain is completely roofed in 

 above. In the teleosts, amphibia, and amniotes no cartilaginous 

 roof (tegmen cranii) is found in this region ; 1 and in lizards, 

 birds, and certain teleosts the trabeculae retain their condition of 

 simple rods closely applied to each other. In most other ver- 

 tebrates the trabeculae gradually grow together beneath the 

 twixt and fore brains, thus forming a complete floor. In the 



1 The history in the Dipnoi is not known. In nearly adult animals (Prolopterus) 

 there exists a longitudinal rod of cartilage in the roof of the skull which may be the 

 remains of an earlier complete cartilage roof. The same may also be true of an isolated 

 cartilage plate in the skull of Polypterus. 



