430 THE ORIGIN OF VERTEBRATES 
mid-ventral surface of the body, which extended from mouth to anus, 
Such a tube might be formed from the mid-ventral surface as follows :— 
In Fig. 163, A, the lining of the ventral surface between two 
appendages is represented flat, in B is shown how the formation of a 
solid rod may arise from the bulging of that ventral surface, and 
in C how a groove on that surface may lead to the formation of a 
tube between the two appendages. The difference between a noto- 
chordal rod formed as in B from that in C would be shown in the 
sheath, for in B the sheath would be formed from the cuticle of the 
lining cells, and in C from the basement membrane. The structure 
of the sheath is in accordance with the embryological evidence that 
the notochord is formed as a tube from a groove, as in C, and not as 
a solid rod as in B, for it possesses a well-marked elastin layer, and 
elastin has never yet been found as a constituent of any cuticular 
secretion, but invariably in connection with basement-membranes. 
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Fic. 168.—D1aGRram OF TWO POSSIBLE METHODS OF THE FORMATION OF A NOTOCHORD. 
The position, then, of the notochord and its method of formation 
suggests that the mid-ventral surface of the arthropod ancestor of the 
vertebrate formed a deep groove between the bases of all the proso- 
matic, mesosomatic, and metasomatic appendages, which was sub- 
sequently converted into a tube extending along the whole of the 
body between mouth and anus, and finally, by the proliferation of its 
lining cells and their conversion into notochordal tissue, became the 
notochordal rod of the vertebrate. 
As already frequently stated, Apus and Branchipus are the two 
living arthropods which most nearly resemble the extinct trilobites. 
The beautiful specimens of Triarthrus (Fig. 165) found by Beecher 
give an idea of the under surface of the trilobite such as has never 
been obtained before, and demonstrate how closely the condition of 
things found in Apus (Fig. 164) was similar to that occurring in the 
trilobites. In both cases the mid-ventral surface of the animal 
formed a deep groove which extended the whole length of the 
