SKELETON. 



139 



ance of a string of beads (Fig. 146). In the amphibia we have 

 at first the conditions just described, and in the perennibran- 

 chiate forms this amphi- 



■4 



;2. 



'\ 



Fig. 146. Diagram of method of growth 

 of amphicoelous vertebrse. c, centrum ; /, in- 

 tercentral enlargements of, n, notochord ; 1-4, 

 successive layers of centra. 



coelous condition persists 

 throughout life. Higher 

 still, there appears an in- 

 tervertebral growth of car- 

 tilage (Fig. 147, yi) which 

 produces a secondary series 

 of constrictions in the no- 

 tochord. A later stage in 

 the process is shown in Fig. 147, B, where an absorption of a 

 part of the intervertebral cartilage is taking place in such a way 

 as to result in the formation of a cup at one end of the vertebra, 

 and at the other of a rounded extremity which fits the cup at 

 the end of the next vertebra. The extreme of the process is 

 shown in Fig. 147, C, where the intervertebral cartilage has been 

 cut conipletely in two, the result being the formation of a ball 



and socket joint be- 

 .0 .,-jSi tween the successive 

 vertebras ; while ossi- 

 fication has extended 

 so far that almost the 

 entire centrum as well 

 as a part of the inter- 

 vertebral cartilage has 

 been converted into 

 bone. When this pro- 

 cess results in a cen- 

 trum rounded in front 

 and hollow behind, 

 we have an opistho- 

 oelous vertebra ; when 

 rounded behind and 

 hollow in front, it is procoelous. A statement of the occurrence 

 of these three types of vertebrae centra may be given here. 



Amphicoelous : most fishes, most perennibranch urodeles, 

 some salamanders, some stegocephali, gymnophiona, many di- 



FlG. 147. Diagrams of developing vertebrae of 

 urodeles, modified from Wiedersheim. c^ centrum; 

 ch, chorda ; e, elastica externa ; i, intercentral carti- 

 lage ; /, ligament ; s, incisure in cartilage. Bone 

 lined, cartilage dotted. 



