ARCHETYPE OF THE SKELETON. 83 



extinct — in wliicb, whilst the peripheral elements of the 

 vertebra become ossified, the central one remains unos- 

 sified ; and here a few words are requisite as to the de- 

 velopment of vertebrae. 



The central basis of the neuro-skeleton is laid down in 

 the embryo of every vertebrate animal, as a more or less 

 cylindrical fibrous sheath, filled with simple cells con- 

 taining jelly. This fibro-cellalo-gelatinous column is 

 called "notochord," Fig. 1, ch (Gr. notos^ black; chorda^ 

 cord; in Latin, "chorda dorsalis"). The centrums, or 

 "bodies of the vertebrae," as anthropotomists call them, 

 are developed in and from the notochord. The bases of 

 the other elements of the vertebra are laid down in fibrous 

 bands, diverging from the notochord, and giving the first 

 indication of the segmental character of the skeleton. At 

 this stage, the skeleton of the little fish called " lancelet" 

 [Amphioxus lanceolatus) is arrested. These fibrous bands 

 are next converted into cartilage, and the cartilage is in 

 definite pieces in each segment, recognizable as "neura- 

 pophyses" (Fig. 1), 7i; "plearapophyses" {ih)^ pl'^ "neural 

 spine" (z5), ns — the centrums still remaining in their 

 primitive state as the undivided notochord {ih\ ch. At 

 this stage, the skeleton of the sturgeon is arrested. The 

 peripheral elements may be converted into bone, the cen- 

 tral ones remaining as notochord, as in the protopterus, 

 the lepidosiren, and many fossil fishes. But, more com- 

 monly, the next stage is the subdivision of the notochord 

 into a series of separate centrums, corresponding with the 

 pairs of neurapophyses and pleurapophyses — ossification 

 of all the parts being more or less imperfect, as in the 

 sharks and rays, which have thence been called " cartila- 

 ginous fishes." When the parts of the vertebrae have 

 become more completely ossified, as in the fishes called 



