314 
DE. CLELAKD ON THE EELATIONS 
With regard to the nasals : to understand the exact morphological part which is played 
hy them, we must have recourse to embryology. The only theory of the segmentation 
of the skull, as far as I know, in which the teachings of embryology have been taken into 
account and been sought to be explained, is that of Professor Goodsie. He is of opinion 
that the maxillaries are developed, not, as has been supposed, in the maxillary lobes, but 
in the lateral frontal processes of Keichaet *; and certainly, if the maxillary lobes are 
homotypic with the pair of visceral laminse behind them, this hypothesis presents the 
only escape from a most serious difficulty; for, while the maxillary lobe arises from 
behind the eye, all theories agree in representing the maxillary bone as belonging to 
a segment in front of the eye. But although it is with the utmost diffidence that I 
would express an opinion differing from Professor Goodsir’s, I take courage to do so in 
the present instance, inasmuch as his view does not agree with the conclusions arrived 
at above from the consideration of adult forms, and because, from observations made on 
embryo lambs, I am convinced that embryologists have been right in considering that 
the maxillary bones are developed from the maxillary lobes. I believe that the true 
solution, and that which will be found to explain all the phenomena, is this ; that the 
cleft between the maxillary lobe of the embryo and the lateral frontal process is not 
transverse in position, but longitudinal ; that it does not separate an anterior from a 
posterior segment, but that it divides the inferior elements of more than one segment 
from the corresponding superior elements. It will be observed that if the maxillary 
bone belongs to a segment in front of the eye, and if it is really developed from the 
maxillary lobe, it follows as a necessary consequence that the nature of the cleft between 
the maxillary lobe and lateral frontal process is as now stated. For on one side of the 
cleft is the blastema in which the palatal and maxillary afterwards appear, while on the 
other side is that in which appear the frontal and nasal, and doubtless also the lateral 
mass of the ethmoid. Moreover, this idea of the fusion of segments by longitudinal 
cleft is by no means an unwarranted assumption, as may possibly be alleged. In sup- 
port of it we observe, — 
1st. The maxillary lobe, as it grows, runs alongside of the lateral frontal and middle 
frontal processes, and does not strike out at right angles to them. 
2nd. The permanent severance of the pterygoid of the fish from the base of the skull 
is an instance of longitudinal fission of at least one sclerotome, remaining throughout life. 
3rd. The hypothesis of fission of segments by longitudinal clefts is necessary to 
explain the peculiar condition of the elements of the face in cartilaginous fishes, if the 
head is to be considered as segmented at all ; for no one can suppose that the upper jaw 
of the cartilaginous fish, and the parts which unite it to the skull, are all parts of one 
segment. According to my hypothesis, the condition is merely an arrestment of deve- 
lopment; for I judge from the maxillary lobe in mammals being originally separate 
from the superior parts of the face, and afterwards united to them, that longitudinal 
fission is a symptom of degeneration at the extremity of the series of segments, which 
* Op. cit. p. 120. 
