spiracular cartilage 

 orbital process 



spina! musculature 



precereb 



pharyngobranchiol 



.epibranchial 

 hyomandibula{epihyal) 



nosal capsule 

 orbitonasal foramen 



labial cartilages' 



A 



ceratohyal parietal foramen 



ceratobranchial 



supraorbital forai 



adductor process 



Meckel's cartilage 



endolymphatic foramei 

 superficial ophthalmic VII and V Q|i(- branches of Vll canalicular foramen 



hyomondibulor articulation 



branch Vll 

 artery 



tery 



B 



palatoquadrate articulations 



pterygoid process 



nasal capsule. 



canalicular foramen 

 endolymphatic foramen 

 cerebellar fossa 



occipitospinal nerve foramina 

 1st spinal nerve 



parietal foramen 

 olfactory foramen 



larotid 

 canal 



subcapsular canal 



VI 

 pituitary vein 



carotid cana 



efferent pseudobronchial artery 



fossa of inferior lobe 



notochord 



palatine branch Vll 

 groove for orbital artery- 



Figure 5-16. Head skeleton of the shark, Squalus. A, lateral view; B, lateral view of endocronium; 

 C, dorsal view; D, ventral view; E, medial view of right half of endocronium. 



EMBRYOLOGiCAL DEVELOPMENT The development of Squalus 

 acanthias has been described by many individuals (Figure 

 5-17). The difficulties of studying stages of development are 

 commented on by Holmgren. Surveying the literature, one 

 gets the impression that investigators have begun their 

 accounts with the stage where the first cartilage appears. 

 In reality, the appearance of the first cartilages is a rather 

 late event in the development of the skull, which is already 

 outlined in blastematic or mesenchymatic stages. Further, 



experience teaches that chondrification begins at very dif- 

 ferent stages in embryos of the same species. Because of this 

 variability it is often difficult to line up stages in a contin- 

 uous series with reference to the development of a particu- 

 lar part. Therefore a detailed comparison of embryos 

 belonging to different groups of selachians must be more or 

 less hazardous. A more general account, however, can serve 

 the purpose of a comparative study. 



The parachordals appear first, well behind the tip of the 



120 • THE HEAD SKELETON OF FISHES 



