THE SKULL. 



467 



basilar plate ; though in a certain number of cases remnants of it are 

 found in the adult state. 



It will be convenient to say a few words here with reference to the 

 notochord in the head. It always extends alotig the floor of the mid- 

 and hind-brains, but ends iniinedi itely behind the infundibulum. The 

 limits of its anterior extension are clearly shewn in fig. 43. The front end 

 of the notochord often becomes more or less vent rally flexed in corre- 

 spondence with the cranial flexure; its anterior end being in some instances 

 (Elasmobranchii) almost bent backwards (fig. 324). 



Kiilliker has shewn that in the Rabbit', and I believe that a more or 

 less similar phenomenon may also be ob- 

 served in Birds, the anterior end of the 

 notochord is united to the hypoblast of 

 the throat in immediate contiguity with 

 the opening of the pituitary body ; but it is 

 not clear whether this is to be looked upon 

 as the remnant of a primitive attachment 

 of the notochord to the hypoblast, or as a 

 secondary attachment. 



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 

 KoUiker as indicative of a segmentation 

 of this }jlate ; but they hardly appear to 

 me to be capable of this interpretation. 



In Elasmobranchs where, as shewn 

 above, a very primitive type of develop- 

 ment of the vertebral column is re- 

 tained, we find that the basilar plate 



is at first formed of (1) the notochord invested by its cartilaginous 

 sheath, and (2) of lateral masses of cartilage, the parachordals, homolo- 

 gous 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 charac- 

 teristic 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 oblongata, 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 beiomes laterally continuous 

 1 "Einbryologische Mittheilungen." Festgchrift d. Naturfor. G<'seZ/., Halle, 1879, 



80—2 



Fig. 324. Longitudinal sec- 

 tion THROUGH THE BRAIN OF A 

 YOUNG PriSTIURUS EMBRYO. 



cer. commencement of the cere- 

 bral hemisphere; im. pineal gland; 

 I;t. infundibulum ; ^jf. ingrowth f ora 

 mouth to form the pituitary body ; 

 mb. mid-brain; cb. cerebellum; ch. 

 notochord ; al. alimentary tract ; 

 laa. artery of mandibular arch. 



