( 309 ) 



TABLE XXXV. 



Systematic Survey of the most Important Pericds in the PnYLOGENY 



OF THE Human Skeleton. 



I. First Period : Skeleton of the Chordonia (Fig. 187, p. 00). 

 Thi' outire skeluton is formed by the notochord. 



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



A iiotoclioid-membrane, the dorsal continnatiou of which forms a cover 

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



III. Third Period: Skeleton of the Ciidostomi (Fig. 190, p. 103). 



A cartilaginous primordial skull develops rouud the anterior extremity 

 of the notochord, from the nntocliord-membrane. An outer cartilaginous 

 gMUskcleton 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 (biserial) 

 skeletons, appear. 



V. Fiftit 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 j^airs of fins becomes 

 uni^erial (semi-pinnate). 



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

 The skull becomes partially ossitied ; 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 rjiys dimini^^h in 

 number to lour, thus givii.g rit>e lo the five-toed f(K)t. The vertebral 

 column ossifies. 



