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. 
