SKELETON 47 



dont whales and in several fossil edentates, but in the living species they occur 

 only in the back and dorsal fins of some cetacea and in the armadillos,where they 

 form a complete armor above, the plates arranged in transverse rows, some of 

 which are movable on each other. In the extinct glyptodons they formed an 

 inflexible case. It is uncertain whether these are a new acquisition in the eden- 

 tates or have been inherited from non-mammalian ancestors. 



THE ENDOSKELETON 



The endoskeleton may pass through three stages in its develop- 

 ment, including the membranous stage (p. 41). From this it may 

 pass through a cartilage stage before becoming bone, or it may in 

 part develop directly into bone from membrane, or, lastly, it may 

 never pass beyond the cartilage stage. Thus only the membranous 

 stage is constant. These differences in development are of great 

 importance in tracing homologies between bones in different groups, 

 but the distinction between bones developing directly from mem- 

 brane (membrane bones) and those passing through a cartilage stage 

 (cartilage bones) can be recognized only by following the ontogeny 

 of the element in question. 



© (O) (O 



Fig. 40. — Diagram of growth of bone. A, from an animal recently fed with madder 

 causing a layer of bone (black) colored by the dye; B, later, no madder fed for some 

 time, a deposit of colorless bone on outside of colored layer, internal layer thinner; C, 

 still later, outer layer thicker, inner layer absorbed. 



As stated above, there is much evidence to show that the membrane bones 

 are dermal bones which have sunk to a deeper position and have become second- 

 arily associated with the endoskeleton. This is especially evident in the skulls 

 of some of the lower ganoids. Ossification of cartilage takes place in two ways. 

 In ectochondrostosis the deposit of lime salts begins on the deeper surface of the 

 perichondrium and gradually invades the cartilage. In entochondrostosis the 

 cartilage becomes altered in the interior, some of the cells becoming modified 

 into osteoblasts, and from these as centres of ossification, the process of bone 

 formation extends in all directions. In ectochondrostosis at least, the centres of 

 ossification may have been derived, phylogenetically, from elements of the der- 

 mal skeleton. 



In ossification the bone is developed in layers, between which the osteoblasts 

 are arranged. In the elasmobranchs the skeleton is frequently strengthened by 

 deposits of lime, but this calcified cartilage differs from bone in that the deposits 

 of lime take the form of polygonal plates and there are no lacunas. 



Many bones increase in length by the addition of epiphyses at the ends. 



