THE SKULL. 



567 



Before the parachordals are formed the anterior end of the notochord has 

 usually undergone a partial atrophy ; and its front end often becomes 

 somewhat dorsally flexed. Within the basilar plate it often exhibits two 

 or more dilatations, which have been regarded by Parker and Kolliker as 

 indicative of a segmentation of this plate ; but they hardly appear to 

 me to be capable of this interpretation. 



In Elasmobranchs where, as shewn above, a very primitive 

 type of development of the vertebral column is retained, we find 

 that the basilar plate is at first formed of (i) the notochord 

 invested by its cartilaginous sheath, and (2) of lateral masses of 

 cartilage, the parachordals, homologous with the arch tissue of the 

 vertebral column. This development probably indicates that the 

 basilar plate contains in itself the same elements as 

 those from which the neural arches and the centra of 

 the vertebral column are formed ; but that it never passes 

 beyond the unsegmented stage at first characteristic of 

 the vertebral column. The hinder end of each parachordal 

 forms a condyle articulating with the first vertebra ; so that in the 

 cartilaginous skull there are always two occipital condyles. The 

 basilar plate always grows up behind (fig. 326, so], and gives rise 

 to a complete cartilaginous ring enveloping the medulla oblon- 

 gata, in the same manner that the neural arches envelope the 

 spinal cord. This ring forms an occipital cartilaginous ring ; in 

 front of it the basilar plate becomes laterally continuous with the 

 periotic cartilaginous capsules, and the occipital ring above 

 usually spreads forward to form a roof for the part of the brain 

 between these capsules. In the higher Vertebrates the periotic 

 cartilages may be developed continuously with the basilar plate 



The trabeculae. The trabeculae, so far as their mere anato- 

 mical 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 differ however from the para- 

 chordals in one important feature, viz. that, except at their 

 hinder end (fig. 323), they do not embrace between them the 

 notochord. 



The notochord constitutes, as we have seen, the primitive 

 axial skeleton of the body, and its absence in the greater part of 

 the region of the trabeculae would probably seem to indicate, as 



