OF THE VOMEE, ETHMOID, AM) INTERMAMLLART BONES. 
315 
tends to disappear as the segments involved become more fully developed* * * § . I believe, 
therefore, that the palatals and maxillaries combine to form the upper jaw of the 
cartilaginous fish (as may be seen to advantage in the Sturgeon), and that the extremity 
of the snout represents the intermaxillaryf ; and that thus, for example, the weapon 
borne by the Sawfish is an intermaxillary bearing teeth. Among mammals, to a cer- 
tain extent similar is the arrangement in Cyclopian monsters : the nose is represented in 
them by a proboscis above the single eye, and the maxillary forms no superior con- 
nexion in front of the eye. 
4th. Tendency to longitudinal fission of the anterior cranial segments is exhibited in 
the permanent duplicity, in many animals, of the vomer and of the intermaxillaries ; also 
in the compression of the septum of the nose, and separation of the segments into two 
alternating sets, as above described. Moreover, the flattening at the base of the skull 
of the substance surrounding the chorda dorsalis, and its division into two trabeculse 
where the chorda ceases, are phenomena, as it appears to me, indicating the same 
thing; and in this last case also the divided parts unite as development proceeds J. 
In deference to the opinions of Professor Huxley §, I may here state that I cannot 
think that the cessation of the notochord is proof of the immediate cessation of segmenta- 
tion. The keenest advocate of the segmentation of the cranium will admit that in the 
face the segment or segments are of a degenerated description. So are those at the 
caudal extremity. But while in the caudal segments the arches degenerate before the 
centra, in the cranium the centra degenerate very soon. It is in harmony with this 
that the chorda dorsalis is one of the first elements to disappear in the cranial segments ; 
while the division of the basis cranii into trabeculae is to be accounted for in the way 
above shown || . 
* In this respect longitudinal and intersegmental fissures resemble one another. The parts of the embryo 
which are developed in immediate contact with the cavity of the ovum, and in which the great systems are 
fully represented, viz. the thorax and abdomen, are those in which the various tissues, the osseous, muscular, 
nervous, and vascular, ultimately exhibit the most complete segmentation ; but it is only in the head and 
neck — an extremity of the embryo in which the systems are not typically developed, (and there it is only in 
the visceral walls, which have in the head and neck the most rudimentary development,) — that segments are 
for a time partially separated by fissures of the blastema. These intersegmental fissures are directly con- 
nected with the rudimentary condition of the parts which they separate, and disappear as the latter become 
more fully developed ; and I have wished in the text to indicate that that also is the case with longitudinal 
fissures. 
t At least that part of the intermaxillary which, as above shown, is in osseous fishes Combined with the 
nasal. 
X The foramen in the basioccipital of Phoca vitulina is doubtless a phenomenon of the same description. 
§ Op. cit. p. 52. 
It In justification of the plan pursued in this communication, in which the main argument is drawn from 
comparative anatomy and then shown to be in accordance with development, I shall here state why I 
cannot go so far as to concur in Professor Huxiet’s doctrine {op. cit. p. 5), that “the study of the grada- 
tions of structure presented by a series of living beings may have the utmost value in suggesting homo- 
logies, but the study of development alone can finally demonstrate them.” "Were that doctrine true, it 
2 u 2 
