SKELETON. 



167 



usually composed of squamosal and postorbital. By alteration 

 in the position or extent of the bones these two fossae may unite 

 into a single temporal fossa, and again, the boundaries between 

 this and the orbit may become broken through, the postorbital 

 arch being imperfect or totally disappearing. (For details see 

 Reptilia.) 



Appendicular Skeleton. The appendages of the vertebrates 

 (fins or limbs) arise as paired outgrowths from the sides of the 

 body, one pair, the anterior or pectoral, arising a short distance, 

 behind the pharyngeal region, the other, or pelvic (ventral) pair, 



FIG. 177. Developing fin of trout, after Corning. /, fin; /, myotomes; , 

 notochord ; the myotomes are seen to be proliferating strands of cells into the fin. 



a little in front of the vent. In the higher vertebrates each 

 limb grows out as a simple bud, but in some elasmobranchs 

 the appendages arise as differentiations of a continuous lateral 

 fold on either side of the body. Into these outgrowths migrate 

 cells derived from the muscle plates (Fig. 177), which are to 



