MORPHOLOGY OF TBLEOSTEAN HEAD SKELETON. 533 



Orbitosplieiioid, alisplienoid, basisphenoid are absent. Con- 

 sequently the cranial cavity opens anteriorly into the orbital 

 region by an extremely wide aperture. 



The Visceral Skeleton. 



Stage I. — The branchial apparatus (PI. 28, fig. 6) at this 

 stage consists of four pairs of simple rods of cartilage (br. 1 — 4), 

 each widely separated from its fellow. Lying iu the ventral 

 space between the members of the first and second pairs is 

 another rod {hr. h.), the homologae of the similarly situated 

 one in the embryo salmon — the copulare commune of Stohr 

 (83, p. 7). Unlike that, however, it does not extend in 

 between the hyoid arches. 



The first pair of branchial arches is the most advanced, the 

 fourth least advanced, and the fifth is as yet quite unrecognis- 

 able. Above the roof of the buccal cavity, i. e. where the future 

 pharyugo-branchials will appear, there is no sign of skeletal 

 rudiments. 



Thus this portion of the visceral skeleton presents, at this 

 stage, exactly the same condition as it does in the youngest 

 salmon described by Stohr (82, p. 2) ; and if, for convenience' 

 sake, we may take the development of the latter as a 

 standard of comparison, it has not attained to the same 

 stage as the parachordal tracts. 



In the hyoid arch all the parts are represented except the 

 hypohyals and basihyal. The lower part consists of a stout 

 bar of cartilage (hy.) representing the future ceratohyal and 

 epihyal, which above is attached by the slender pro-carti- 

 laginous stylohyal to the upper part, and below is separated 

 from its fellow by a mass of undifferentiated tissue. 



In the upper part the hyomaudibular (hym.) and sym- 

 plectic (iS-'/m.) regions are not differentiated by an intervening 

 area of pro-cartilage, as in the salmon, but together form a 

 continuous chondrified plate. To the middle of the hinder 

 border of this the stylohyal is attached. This point of con- 

 nection serves to denote the boundary between the hyo- 



