94 WILLIAM BATBSON. 



considerations, it could hardly have been supposed that the 

 head of a three-day chick, for example, was a highly segmented 

 structure, seeing that the regular segmentation of the body 

 conspicuously stops at its junction with the trunk. No doubt 

 the cranial nerves may, by arbitrary divisions and combinations, 

 be shaped into an arrangement which more or less simulates 

 that which is supposed by some to have been present in the 

 rest of the body, but little is gained by this exercise beyond 

 the production of a false symmetry.] 



The Axial Skeleton. — The notochord of the Enterop- 

 neusta is so partially developed that it is not difficult to con- 

 ceive that its presence in the middle third of the body may 

 indicate a stage in its phylogenetic appearance. If while in 

 this condition it was used as a fulcrum in swimming it seems 

 further conceivable that if this organ grew backwards the con- 

 dition of the Ascidian Tadpole's tail would be produced, 

 though no stress can be laid on this view. As will be shown later 

 on, it is likely for other reasons that the Ascidians separated 

 themselves from the other Chordata before Amphioxus, or 

 even the Enteropneusta. 



By extending the separation of the notochord the condition 

 of Amphioxus is reached. And next, the axial column of the 

 Marsipobranchs shows us the notochord enclosed in a meso- 

 blastic sheath as yet unsegmented. This process is fore- 

 shadowed by the presence of rings round the neural canal, 

 placed between the nerves whose segmentation they follow. 

 Finally, in the other Vertebrata the column itself is segmented, 

 so that this is another instance of the appearance of a typical 

 segmentation in a system of a Vertebrate whose origin within 

 the limits of the group is unmistakeably traceable. 



The Myotomes. — Intermediate conditions between the 

 condition of the muscles of Balanoglossus and of Amphioxus 

 are as yet unknown. I submit, however, that it is not im- 

 possible to conceive the formation of myotomes by a simple 

 mechanical process of gathering the muscular fibres into 

 bundles. Their origin as archenteric pouches may then be 

 supposed to have originated from the fact that the ancestral 



