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; n, 

 notochord ; tlie 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 



