316 
DE. CLELAND ON THE EELATIONS 
Conceiving, then, the development of the anterior part of the cranium to take place 
in the manner above stated, I consider that the nasals have the same relation to the 
segment to which they belong as the frontal has to its segment, inasmuch as they are 
continuous in position with the frontal, except in those lower vertebrata in which the 
prefrontals complete a well-developed intervening arch. They are more closely con- 
nected with the corresponding elements behind them than with the remaining elements 
of their own segment, as a natural consequence of the fission of segments. And now it 
will be understood how it is that the nasals are often so far sundered from the inter- 
maxillaries : it is, that the preceding segment having a very elongated centrum (the 
vomer) and a very imperfect neural arch (the lateral masses of the ethmoid), the inter- 
maxillaries are projected forwards in front of the vomer, while the nasals chng behind 
to the frontals. 
With respect to the expanded form of the nasals in many reptiles, in birds, and in 
mammals, I apprehend that they are not to be considered as spreading downwards to 
form an imperfect arch of the neural series, but as spreading outwards to protect two 
intersegmental passages ; for it is the nostrils, and not, in any sense, a continuation of 
the cranial cavity, which they protect. This will be best understood by looking at the 
Frog’s skull, in which the osseous cranial cavity is closed-in at the fore part of the pre- 
fi’ontals, and the neural arch of the segment in front is reduced to zero, the position 
where it exists in certain fishes being represented in the Frog by the line of junction of 
the nasals witl i one another and with the mesial cartilage below ; while the expansions 
of the nasals over the nostrils are like the projections outwards of two transverse pro- 
cesses, and are strictly similar to those elongated projections of the frontals which roof-in 
would be impossible finally to demonstrate the correspondence of any two structures in different animals, 
since we do not see one developed out of the other. But, on the contrary, there is a vast number of 
such correspondences so plainly obvious as to need no demonstration. That which makes these corre- 
spondences so evident is simply the comparison of the bones ; and it is the business of the anatomist, 
in cases of correspondence less obvious, to submit the structures which he compares to a careful scrutiny, 
until, by minute examination of their relations, and determining wliat is constant and what is variable, 
he is able to give a certain judgment upon points which appear to the uninitiated eye obscure. But if 
the prosecution of such researches renders certain the correspondence of a number of elements in different 
animals, will not the light thrown by the varied relations of these elements upon the laws which regulate 
their arrangement furnish as certain and secure data on which to build as can be obtained from the use 
of any scientific instrument whatever ? It has not been the instriunents used, but the manner in which 
they have been handled, that has led to the discrepancies of morphological theories. I have an interest to 
insist on this matter, because it is impossible that the questions discussed in the present communication can 
be settled by embryological evidence, for these reasons : viz. the maximum segmentation of the cranium is 
found, not in the embryo, but at the period of most characteristic development of the bones composing it ; 
and the whole history of the sclerotomes to whose elucidation this communication is devoted is not given 
in any single species ; but, on the contrary, were we to search in the mammal for the stages illustrated in 
the structure of fishes, we should find that at the period at which these stages would faU due to be repre- 
sented, there is as yet no differentiation of tissues, and the osseous system whose history we seek to ti-ace 
has not begun to appear. 
