82 VERTEBRATE SKELETON 



front of the basal plate, hence the trabeculse (morphologically ventral 

 to the brain) are directed downwards, instead of being horizontal 

 as in the general statement above (p. 59), and in Amphibia. Dorsal 

 to and at some distance from the trabecula is a second cartilage, the 

 sphenolateral (pleurosphenoid, alisphenoid), nerves II to V passing 

 between trabecula and sphenolateral, while nerves VI and VII are 

 dorsal to the trabecula and posterior to the hinder end of the upper 

 cartilage. The otic capsule begins as an elevation of the dorsal 

 side of the basal plate, gradually extends dorsally and then medially 

 so that the otic vesicle becomes enclosed in cartilage, except on the 

 medial side where openings for nerves and for ducts (endo- and peri- 

 lymph) remain open permanently. There is no fenestra on the 



Fig. 86. — Early chondrocranium of Acanthias (Sewertzow, '99). The brain in 

 outline, als, sphenolateral; ch, anterior part of notochord; h, hyoid arch; ma, mandi- 

 bular arch, not yet divided; oc, otic vesicle; t, trabecula; 1-5, branchial arches. (Com- 

 pare with fig. 65.) 



external surface at any time. The posterior part of the basal plate 

 is formed by rudiments of occipital vertebras (p. 62), the number of 

 which varies, according to author and species, from four to seven. 



Later these vertebrae form neural arches, the first joining the 

 posterior cupula of the otic capsule, leaving the jugular foramen 

 between the two, while between the more posterior arches (which 

 fuse) are similar foramina for the spino-occipital nerves. These 

 arches meet dorsally and, with upgrowths from the otic capsules, 

 form the synotic tectum which is perforated for the external openings 

 of the peri- and endolymph ducts. 



Each trabecula fuses with the basal plate in front of the otic 

 capsule, and a little later the anterior trabecular ends unite to form 

 the ethmoid plate, enclosing the hypophysis in a large hypophysial 

 fenestra. Trabecula and sphenolateral plate become united by 



