iv^III.] THE TRABECULiE. 239 



The trabeculse. The trabeculse, so far as their 

 mere anatomical relations are concerned, play the same 

 part in forming the floor for the front cerebral vesicle 

 as the parachordals for the mid- and hind-brains. They 

 dififer however from the parachordals in one important 

 feature, viz. that, except at their hinder end, they do 

 Qot embrace between them the notochord. 



The notochord constitutes, as we have seen, the 

 Drimitive axial skeleton of the body, and its absence in 

 he greater part of the region of the trabeculse would 

 jrobably seem to indicate, as pointed out by Gegen- 

 baur, that these parts, in spite of their similarity to 

 he parachordals, have not the same morphological 

 significance. 



While this distinction between the parachordals and the 

 irabeculoe must be admitted, there seems to be no reason against 

 iUpposing that the trabeculse may be plates developed to support 

 ;he floor of the fore-brain, for the same physiological reasons 

 hat the parachordals have become formed at the sides of the 

 Qotochord to support the floor of the hind-brain. By some 

 anatomists the trabeculEB have been held to be a pair of branchial 

 bars ; but this view has now been generally given up. They 

 have also been regarded as equivalent to a complete pair of 

 Qeural arches enveloping the front end of the brain. The primi- 

 tive extension of the base of the fore-brain through the pituitary 

 space is an argument, not without force, which has been appealed 

 o in support of this view. 



In the majority of the lower forms the trabeculse 

 arise quite independently of the parachordals, though 

 the two sets of elements soon unite ; while in Birds 

 (Fig. 76) and Mammals the parachordals and trabeculse 

 are formed as a continuous whole. The junction be- 



