6o VERTEBRATE SKELETON 



below the developing ear (otic vesicle). The two parachordal 

 plates form the floor of the hinder part of the cranium, and, increasing 

 in thickness, the plates meet, at first below, later above the notochord, 

 so that a basal plate is formed, the notochord included in the 

 cartilage. 



It is a question as to how far the basal plate is segmental. In the early 

 stages the part behind the level of the otic vesicles is often metameric in outline, 

 while the union of the parachordals ventral to the notochord recalls the hypo- 

 chordals of vertebrae, possible indications that here are coalesced vertebrae, 

 the number of which varies in difTeretjt Vertebrates. The actual vertebral 

 arches of the postotic region are considered below. 



The basal plate extends as far forwards as the tip of the notochord. 

 A little later than its appearance two longitudinal bands of cartilage 

 develop on either side, beneath and lateral to the brain and in front 

 of the basal plate. The lower pair of these, the trabeculae cranii, 



Fig. 66. — Diagram of developing skull of Acanlhias. bp, basal plate; c, trabecular 

 cornu; //, foramen lacerum; h, hyoid arch; j, jugular foramen; w, Meckel's cartilage; 

 n, roof and wall of nasal capsule; oc, otic capsule; of, optic fissure; ov, occipital vertebras; 

 pp, preotic pillar; pq, pterygoquadrate; si, sphenolateral cartilage; /, trabecula; 1—5, 

 branchial arches. 



are slender bars which lie, one on either side of the hypophysis and 

 ventral to all preotic nerves. As they increase in length they meet 

 in front of the hypophysis, forming by their junction a horizontal 

 cartilage, the ethmoid plate, beneath the anterior end of the brain. 

 At about this time the hinder end of each trabecula fuses with the 

 antero-lateral angle of the basal plate; trabeculse, ethmoid and basal 

 plate forming a ring around the hypophysis, the opening being the 

 h5T)ophysial fenestra. 



The more dorsal bars in front of the basal plate are the spheno- 

 laterals' (fig. 66). They parallel the trabeculae, but he dorsal to the 



1 These are variously called alisphenoid cartilages and pleurosphenoids and are 

 represented in part in the higher groups f Amniotes) by the marginal taenia. 



