38 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



forming an envelope around the internal structures. This connects 

 in the middle line, above and below, with a longitudinal partition which 

 separates the muscle masses of the two sides. This partition splits 

 to pass on either side of the central nervous system and the notochord, 

 and, just beneath the peritoneum, around the viscera. From the 

 median partition sheets of mesenchyme (myosepta) pass vertically 

 between the myotomes to the dermal layer, they being, like the myotomes, 

 metameric. Then there is a horizontal sheet on either side which lies 

 between the epaxial and hypaxial muscles (p. 127). Not all parts of 

 this membranous skeleton develop hard structures, but these are most 

 apt to arise at the intersection of the various planes. 



The skeletal structures are divided into the dermal, arising in the 

 outer mesenchymatous envelope, and the endoskeleton, formed in 

 the other parts and lying deeper in the body. The dermal skeleton 

 includes the scales of fishes, the dermal armor of many reptiles and 

 fossil amphibians and the bony scales in the skin of crocodilians and 

 some mammals. In the strict sense the so-called membrane bones of 

 the skull and the cleithrum of fishes and the clavicle and episternum 

 of higher vertebrates should be included here, since they apparently 

 have been derived from dermal ossifications, but convenience of treat- 

 ment necessitates their consideration with the endoskeleton, with which 

 they are intimately associated. 



It is a question whether the dermal or the endoskeleton is the older. The most 

 primitive of the living species, the cyclostomes, have no exoskeleton, but have 

 cartilage developed to some extent. In development, also, cartilage always ap- 

 pears before there is a trace of the exoskeleton. On the other hand, some of the 

 oldest fishes known have a well developed dermal armor, while the best preserved 

 ostracoderms show no trace of an internal skeleton. The external skeleton has 

 probably arisen as a means of protection, the internal as a result of muscular or 

 other strains. 



Bones are connected (articulated) with each other in different ways- 

 They may be so articulated that one can move on the other (diar- 

 throsis) or there may be no motion possible (synarthrosis), each with 

 several varieties. Of the immovable joints there may be sutures, 

 where the two bones are connected by the interlocking of saw tooth-like 

 projections, or the two may be united by bony growth (anchylosed) 

 so that the line between the two disappears. In those cases of diar- 

 throdial joints where there is much motion there is usually a closed sac, 

 lined by a synovial membrane between the two bones. This mem- 

 brane secretes a fluid which lubricates the surfaces. 



