568 



THE TRABECUL^. 



pointed out by Gegenbaur, that these parts, in spite of their 

 similarity to the parachordals, have not the same morphological 

 significance. 



pts 



FlG. 325. VIEW FROM ABOVE OF THE INVESTING MASS AND OF THE TRABECUL/K 

 OF A CHICK ON THE FOURTH DAY OF INCUBATION. (After Parker.) 



In order to shew this, the whole of the upper portion of the head has been sliced 

 away. The cartilaginous portions of the skull are marked with the dark horizontal 

 shading. 



cv i. cerebral vesicle (sliced off); e. eye; nc. notochord ; iv. investing mass; 

 9. foramen for the exit of the ninth nerve ; cl. cochlea ; hsc. horizontal semicircular 

 canal; q. quadrate; 5. notch for the passage of the fifth nerve; Ig. expanded anterior 

 end of the investing mass ; pts. pituitary space ; tr. trabeculte. The reference line tr. 

 has been accidentally made to end a little short of the cartilage. 



The nature of the trabeculae has been much disputed by morphologists. 

 The view that they cannot be regarded as the anterior section of the 

 vertebral axis is supported by the consideration that the forward limit of the 

 primitive skeletal axis, as marked by the notochord, coincides exactly with 

 the distinction we have found it necessary to recognise, on entirely indepen- 

 dent grounds, between the fore-brain, and the remainder of the nervous axis. 

 But while this distinction between the parachordals and the trabeculae must 

 I think be admitted, I see no reason against supposing that the trabecula? 

 may be plates developed to support the floor of the fore-brain, for the same 

 physiological reasons that the parachordals have become formed at the sides 

 of the notochord to support the floor of the hind-brain. By some anatomists 

 the trabeculaa 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 equiva- 

 lent to a complete pair of neural arches enveloping the front end of the brain. 

 The primitive extension of the base of the fore-brain through the pituitary 



