DEVELOPMENT OF THE SKULL OF THE COMMON FOWL, 
767 
a free descending ray ; but I am extremely doubtful about the latter interpretation, and 
strongly incline to regard the mandibular, pterygo-palatine, and trabecular rays as true 
serial counterparts. 
The true histological growth and development of the pterygo-palatine bar has cost 
me much labour ; only by examining sections made at many stages, and by seeing these 
under very high powers, has the meaning been made apparent. In a fresh-made section 
at this stage (Plate LXXXI. fig. 6) the tissues composing the ethmoid ( eth .), the rostrum 
(r.st.), the palatine (pa.), and the sclerotic (scl.) do not appear very much unlike when 
seen by low powers ; afterwards their differing characters may be seen. The cells com- 
posing the sclerotic (scl.) are smaller than those which make up the ethmoid (eth.) ; but 
they rapidly form hyaline cartilage ; and both these tracts are more translucent than 
the rostral stroma or the cells composing the palatine. The latter can, however, be seen 
to transmit the light better than the rostral mass ; and by the use of high powers (320 
diam.) the structure of the rod becomes sharply distinguished from the nascent cellular 
tissue in which it is imbedded, and also from the more advanced connective-tissue cells 
of the rostrum. Much the greater part of the palatine bar (fig. 11) is composed of 
delicate, sharp-ended, fusiform cells of simple cartilage*, having large granular nuclei ; 
but the pith of the bar is composed of still more delicate and smaller cells having an 
oval form, and without apparent nuclei — at least as seen magnified 320 diameters ; it is 
this pith which is the first to ossify ; and the transformation of the simple cartilage into 
bone is so rapid as to give the deposit the appearance of a membrane bonef . 
The further development of the palatine bar will be described hereafter ; and the 
pterygoid (p.g .), which is at present composed of simple cartilage similar to that of the 
palatine region, will be considered at the same time. 
There now remain for description the free part of the facial (poststomal) arches ; and 
the keystones that complete them. There is no keystone to the mandibular arch, which 
* Professor Huxley is disposed to look upon what I call “ simple cartilage ” as merely “ indifferent tissue.” 
f For many years the development of the pterygo-palatine har in the Sauropsida and Mammalia has been 
a great difficulty to me, as with moderately high powers, at stages more advanced than the one under notice, 
I could not well see how the main part of the har differed from a membrane bone ; yet I had discovered the 
existence of hyaline cartilage in several parts of the har in Birds. In the Passerinse and some others, the broad 
transpalatine angle is not ossified by the proper palatine centre, but by a separate endosteal deposit, and that 
long after the main bar, so that the simple cartilage gains time for a plentiful intercellular deposit. In the 
Woodpecker ( Picus viridis) the inner edge of each palatine does not ossify from the main bony rod, but 
developes into hyaline cartilage, coalesces with its counterpart of the other side, and then slowly and feebly 
ossifies by endostosis, this centre being, properly speaking, an azygous “ mesopterygoid.” Hyaline cartilage 
may be found at the pterygoid end of the palatine proper in certain Birds (e. g. Gaprimulgus) ; then it also 
appears in the counterpart facet in the pterygoid ; it is always present on the quadrate end of the pterygoid ; 
and whenever this hone articulates with a basipterygoid process, a meniscoid plate is developed on the corre- 
sponding part of the pterygoid. In many Reptiles (Lacertians and Chelonians) the posterior part of the sub- 
ocular cartilage chondrifies and developes into a straight rod, which is obliquely situated above the fibrous or 
main pterygoid ; ossifying separately, it becomes the so-called “ columella : ” the pterygoid of the higher Birds 
is more nearly related to this than to the underlying fibrous hone. 
