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TABLE XXXV. 



Systematic Survey of the most Important Periods in the Phylggkny 



OF THE Human Skeleton. 



L First Period : Skeleton of the Chordonia (Fig. 187, p. 90). 

 The entire skeleton is foiined by the notochord. 



II. Second Period : Skeleton of the Acrania (Fig. 189, p. 91). 



A notocliord-niembrane, tlie dorsal continuation of which fomui a cover 

 ing roand the medullary tube, is formed round the notochord. 



in. Third Period : Skeleton of the CyclostonU (Fig. 190, p. 103). 



A cartilaginous primordial skull develops round the anterior extremity 

 of the notochord, from the notochord-membrane. An outer cartila^nous 

 gill-skeleton forms round the gills. 



IV. Fourth Period : Skeleton of the older Selachii (Fig. 268, p. 302). 



A primitive vertebral column, with upper and lower arches (the gill- 

 arches and ribs) forms round the notochord. The remnant of the outer gill- 

 skeleton remains with the inner. Two pairs of limbs, with pinnate (biaerial) 

 skeletons, appear. 



V. Fifth Period : Skeleton of the more recent Selachii (Fig. 269, p. 302). 



The anterior gill-arches change into lip-cartilage and jaw-arches. The 

 external gill-skeleton is lost. The skeleton of the two pairs of fins becomes 

 uniserial (semi-pinnate). 



VI. Sixth Period : Skeleton of the Dipneusta (Fig. 2, Plate XH.). 

 The skull becomes partially ossified j as does the shoulder-girdle. 



VII. Seventh Period : Skeleton of the Amphibia (Fig. 270, p. 302). 



The gill-arches are modified into parts of the tongue-bone, and of the jaw. 

 apparatus. On the semi-pinnate skeletons of the fins the rays diminish in 

 number to four, thus giving rise to the five«toed foot. The vertebral 

 oolumn ossifies. 



