42 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



nervous system and the notochord, and, just beneath the peritoneum, 

 around the viscera. From the median partition sheets of mesen- 

 chyme (myosepta) pass vertically between the myotomes to the 

 corium, they being, like the myotomes, metameric. Then there is 

 a horizontal sheet on either side which lies between the epaxial and 

 hypaxial muscles (p. 132). 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 d ermal skeleto n 

 includes the scales of fishes, the derm j.1 arm or of many reptiles and 



fossil amp hibi ans 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 epi- 

 sternum of higher vertebrates should be included here, since they 

 apparently have been derived from dermal ossifications, but con- 

 venience of treatment 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 dermal skeleton, 

 but have cartilage developed to some extent. In development, also, cartilage 

 always appears 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 and cartilages are connected (articulated) with each other in different 

 ways, the parts in some joints being movable on each other; in other cases no 

 motion is possible. In the latter (synarthroses) there may be a close approxima- 

 tion of the elements with a thin membrane between them, this type being 

 strengthened in the skull by the interlocking of sawtooth-like projections 

 (suture). In places, with growth, all hnes of demarcation between two originally 

 distinct elements may disappear and the elements form one continuum (ankylo- 

 sis or synosteosis). The movable joints are grouped under two categories. 

 In amphiarthrosis, illustrated by the movements of the human vertebrae on each 

 other, the motion is slight and is accomplished largely by intervening ligaments 

 and cartilages. In diarthrosis the parts are freely movable, there being between 

 them a closed articular cavity, lined by a (synovial) membrane which secretes 

 a fluid which lubricates the surfaces. 



Cartilages and bones are covered on their outer surfaces by an 

 envelope of connective tissue, called respectively perichondriiim or 



