eye muscles enter 

 anterior myodome / 1^ 



olfactory nerve (I) 



lateral ethmoid 



nerves ond blood vessels 

 interorbital septum 



optic pedicel 



Figure 5-6. Snout part of the adult endocranium showing the anterior 

 myodome; the roof of the right half has been cut away (cross hatching 

 indicates surfaces). 



canal. In still other bones, the basal plate appears at much 

 the same time as the canal organ center. 



In contradiction to the concept of sensory organ of tooth 

 origin of bones is the fact that not all bones of the fish arise 

 in relation to one of these sources; further, there is experi- 

 mental evidence that bones will develop, somewhat later 

 perhaps, after the surgical removal of the sensory organs. 

 Also, in many animals sensory organs are never developed 

 but bones arise much as the basal plate of the fish canal 

 bone. It has long been assumed that the sensory organ, as 

 a more differentiated structure lying in a canal and enclosed 



by a concentration of connective tissue, presents an area 

 with a higher metabolic rate and substrate conditions that 

 might serve as a first condensation point. There is little 

 question that in fishes the sensory organ centers are char- 

 acteristic and useful in determining homologies. 



The origin of the maxilla and dentosplenial is of sjjecial 

 interest. A strand of mesenchyme extends out and back from 

 the anterior end of the palatoquadrate (Figure 5-9). This 

 strand posteriorly is associated with Meckel's cartilage and its 

 end grows down and forward with that structure. The strand 

 now lies in both upper and lower lip margins and appears to 

 give rise to the maxilla above and the dentosplenial below. 

 Anterior to the tip of the maxillary blastema, a premaxil- 

 lary blastema condenses from the mesenchyme underlying 

 the snout. Behind this, pairs of vomer and palatine anlages 

 appear. Behind the dentary, an angular blastema forms. All 

 of these blastemas are similar and represent bone origins 

 that cannot be directly associated either with teeth or sen- 

 sory structures. A small quadratojugal appears and fuses to 

 the quadrate. 



The internasal and vomer have chondral relationships; 

 the former may be fused to the mesethmoid. The basi- 

 sphenoid arises membranously in Salmo and has three centers: 

 a ventral midline and bilateral dorsolateral centers. In some 

 teleosts the basisphenoid has a cartilaginous precursor. This 

 cartilage must be viewed as secondary. The intercalare is 

 basically an ossification in a ligament, not a primary com- 

 ponent of the endocranium. The chondral bones all arise 

 perichondrally. 



The hyomandibula arises in a way comparable to that 



nternal rectus 



hyomandibular 

 branch VII 



trabeculo communis, 

 basisphenoid ligament^ 

 optic ar^ery^ 

 cerebral artery 

 ophthalmic artery, 

 palatine branch of VM^ 

 pituitary vein 

 efferent pseudobranchial artery 

 orbital artery. 



trigeminal (V) root. 



lateral head vein 



left radix aorta 



nferior rectus 



.oculomotor nerve (III) 

 superior rectus 



external rectus 



.facial foramen 



membranous prootic bridge 

 ucens nerve ( 



outline (dashed) of pituitary body 



external rectus in myodome 

 notochord 



Figure 5-7. Migration of eye muscle origins through the hypophyseal fenestra and the beginning 

 of the myodome in a 22-mm larval salmon. (After Goodrich, 1930) 



GNATHOSTOME FISHES 



111 



