762 
ME. W. K. PAEKEE ON THE STEUCTTJEE AND 
eventually becomes the fore part of the “ tentorium cerebelli.” The internal carotid 
artery ( i.c .) is seen to be entering the skull in front of this wall. 
The greater part of the investing mass may now be treated of as the basioccipital 
region ( b.o .) ; anteriorly the nascent fibrous tissue is thickening below this part 
to become a bed for the deposit of the basitemporal. 
The pituitary space is now much foreshortened, very deep, and is indicated in the 
figure by the entrance of the internal carotid artery (i.c.) ; the cartilage has become high, 
even below and between the optic nerves (2) ; and close in front of the pituitary body 
the anterior clinoid ridge ( a.cl .) has commenced. But in front of the optic nerves the 
greatest and most sudden change has taken place, the very short broad waist between 
the pituitary space and the rudimentary ethmoidal alee shown in fig. 2 being at this stage 
greatly elongated and altogether developed into a high crest : this is the ethmo-pre- 
sphenoidal plate (p.sp., eth.). This plate, which is rapidly passing into hyaline carti- 
lage, is somewhat higher behind than before, is nearly straight above, is oblique behind, 
above the optic nerves, concave below, and has a gently convex margin in front. 
The alae of the nasal labyrinth are not shown in this section ; but above and below 
the line where these have been cut away on the left side, two projections are seen. 
The upper of these is the retral spike, in which the ethmoid ends behind above 
the olfactory crura* ; whilst the lower process is one of the greatest importance in the 
morphology of the face, it being the prenasal or snout-cartilage (p.n.). This part, 
which is the endoskeletal axis of the premaxillary region, grows at first backwards as 
well as downwards, having the same direction as the “ fronto-nasal lamina,” and being 
an azygous outgrowth from the crest which forms between the trabecular cornua — the 
rudimentary septum nasi, continuous with the ethmo-presphenoidal plate. The cornua 
themselves, as I have already asserted, develope themselves into the alee nasi. This con- 
dition of the prenasal cartilage is like what is seen in the ripe embryo of the Green 
Turtle (Chelone mydas) ; in that animal, however, it is less rounded in shape, and projects 
further downwards, forming a soft core to the premaxillariesf. The thick arcuate 
inferior edge of the ethmo-presphenoidal plate has already begun to be underlain by 
that fibrous stroma which afterwards thickens very much between the basis cranii and 
palatal skin to form a nidus for the rostral part of the “ parasphenoid.” 
Below, and on each side of the basis cranii, the maxillary rudiment has begun to send 
downwards the facial lamina in which the maxillary and jugal bones are afterwards de- 
veloped, and to have a more distinct rod on the inner side, which becomes the palatine 
(pa.) and the pterygoid (p.g .) ; but this will be best studied in a chick a day or two 
* This spike is called “ crista galli ” ( cr.g .) in my former paper ; but Professor Huxley has shown me 
recently that there is nothing in the Bird which answers to that plate, save that notched and grooved part 
of the perpendicular ethmoid which lies exactly between the olfactory nerves ; so that, practically, it may 
be said to be undeveloped in this Class. 
t Professor Huxley is strongly inclined to believe that the “ trabeculae,” from their roots, are in reality the 
foremost (and of necessity the innermost) pair of visceral arches. 
