52 THE VERTEBKATE SKELETON. 



condition corresponding to a very early stage in Vertebrata. 

 The skeleton of Amphioxus is partly hypoblastic, partly meso- 

 blastic in origin. 



(a) Hypoblastic skeleton. 



The notochord (fig. 3, 2) is an elastic rod extending along 

 the whole length of the body past the anterior end of the 

 nerve cord. It lies ventral to the nerve cord, and shows no 

 trace of segmentation. It is chiefly made up of greatly vacuo- 

 lated cells containing lymph, but near the dorsal and ventral 

 surfaces the cells are less vacuolated. The notochord is im- 

 mediately surrounded by a structureless cuticular layer, the 

 chordal sheath, and outside this comes the mesoblastic skeleto- 

 genous layer, which also surrounds the nerve cord. 



The branchial skeleton. This consists of a series of 

 chitinous elastic rods which strengthen the gill bars and are 

 alternately forked and unforked ventrally. The forked rods 

 are primary, and are U-shaped in section, the unforked rods 

 are secondary, and are circular in section. All these rods are 

 united at intervals by transverse rods. 



(b) Mesoblastic skeleton. 



The buccal skeleton. On each side of the mouth there 

 is a curved bar resembling the notochord in structure. The 

 bars are segmented, and each segment bears a smaller rod 

 which supports a tentacle, the whole forming the buccal 

 skeleton (fig. 3, 4). 



The notochord is enclosed in a thick sheath of connective 

 tissue continuous with a thinner sheath round the nerve cord. 

 The sheaths of the notochord and nerve cord together form the 

 skeletogenous layer, and prolongations of it form the myomeres 

 or septa between the myotomes or segments of the great lateral 

 muscles of the body. 



The skeleton of each median fin consists of small 

 cubical masses of a gelatinous substance arranged in rows 

 (fig. 3, 1 and 7), and serving to strengthen the fins. 



