562 WILLIAM BATESON. 
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 
