436 



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. 





A J3 C 



Fig. 1G3.— Diagram 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 Triartbrus (Fig. 165) found by Beecher 

 sjive 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 



