.984 MR. ST. GEORGE MIVART ON THE VERTEBRATE SKELETON. 
to the eostal arches; but the main trunks of nerve are outside these, not on their abdo- 
minal aspects, as they would have been were August Müller's theory correct.” 
Dr. Cleland regards the costal arches of fishes as homologous with those of the higher 
vertebrata; but I do not find a distinct affirmation of any general relationship between 
the subeaudal arches of fishes and chevron bones or other parts of higher animals. 
But he does not appear to coincide with my view, as he denies the homology between 
what I believe to be the same parts in Menobranchus and Naseus fronticornis * and 
asserts that the subcentral arches of the trunk-vertebre of the Sole are of essentially the 
same nature as transverse processes. 
Professor Huxley has not, so far as I know, given forth any decided view as to the real 
nature of the subcaudal piscine arches. For myself, I believe that the facts will not 
warrant us in saying that they belong altogether to any one category, but that their na- 
ture is more or less different in different cases. 
A priori we might expect that in these lower vertebrata “ differentiation ” in some 
parts of the skeleton would be less complete than in higher classes; and such I believe 
to be the case as regards the hypaxial and paraxial elements. 
In the embryo before the bifurcation of the laminæ of the ventral fold is complete, it 
can only be approximately determined to which category portions of tissue belong which 
are situated in the as yet undifferentiated part. And if, by arrest of development, the 
bifurcation never extended further, hard parts so situated could only be so determined 
during the whole of life. But as the alimentary tube (except of course its mucous lining) 
is undoubtedly formed from the internal laminæ, the parts lying directly above it and . 
beneath the vertebral column may well be taken to belong to the same category, lying, 
as they do, at least in the roots of such laminæ and above their points of suspension. 
Such skeletal elements in the trunk, when they coexist with distinct paraxial elements 
surrounding externally the visceral cavity, being deemed hypaxial, the nature of the 
subcaudal arches must be determined by examining whether they are serially related 
to one or the other set of elements. 
That they should partake of the nature of both systems and be parhypaxial, or that 
distinct paraxial ribs should start from undifferentiated parhypaxial root-processes, 
would only harmonize with what we might expect « priori to find in some cases, espe- 
cially in members of the lowest class. We have seen that in some vertebrates, upon an 
hypothesis suggested a little back, coalescence or connation takes place between parts 
which are distinct in other forms (i. e. the coalescence between diapophysial and para- 
pophysial elements); it is therefore not improbable that we may find a similar want of 
differentiation (coalescence or connation) with regard to such skeletal elements as the . 
paraxial and hypaxial ones. 
Again, the internal and external laminæ of each ventral plate merge one into Pei other 
superiorly; and we might expect sometimes to find solid skeletal parts in certain situa- 
tions, the true and essential nature of which is onlv be determined by following it 
serially in the same individual, or by carefully comparing it with other forms. 
From the piscine skeletons I have as yet examined, I believe that in the great Mons 
* Loc. cit. p. 131. 
