542 
ME. W. K. PAEKEE ON THE STEUCTITEE AND DEVELOPMENT 
The auditory sac (Plate 22. fig. 3, au.) shows two masses of otoconial crystals, and its 
floor is undergoing chondrification. 
If the primordial trabeculae have to be eliminated from the pleural category, there is 
nothing of the kind in front of the mouth at present. But as a long series of more and 
more imperfect vertebral segments are developed in various types beyond the secondary 
anal opening, so it is no wild search to be looking for rudiments in front of the secondary 
mouth. These will be discussed in the succeeding stages. 
In larvae less than half an inch in length (5^ lines) a still further advance is to be 
found (Plate 23) ; but there is no new element*. 
The cephalic part of the notochord (Plate 23. figs. 1 & 2, nc.) is still large, and full half 
of it in front is invested by trabecular cartilage. These rods have grown much back- 
ward and upward, and but little forward. They are now composed of solid cartilage. 
They only embrace the . sides of the thalamencephalon, and bound it in behind, not quite 
meeting, being kept apart by the apex of the notochord. 
Behind these expansions there is no cartilage, the whole occipital ring being still 
membranous, and the two muscular segments {ms.) being visible from above when the 
hind brain is removed. 
The hind brain is now walled in at the sides by the great trabecular crest (figs. 1, 5, 
& 6, tr.), the height of which is equal to the breadth of the broadest part of the bar 
whence it arises. The temporal muscle (fig. 6) arises from this crest, and is inserted into 
the coronoid region of Meckel’s cartilage. 
There is no distinction between alisphenoid and orbito-sphenoid either in Amphibians 
or Selachians ; and there must be some meaning shut up in the fact that the greatest 
similitude to vertebral segments is seen in the most specialized types of skull. The 
trabeculae are oval on section in front (fig. 4, tr.), and end opposite the eyeballs on the 
side, and the junction of the mid and fore brain within. At present, then, the 
ethmoidal and nasal regions and the lateral “ horns ” are absent. Only the common, 
paired rudiments of the posterior and anterior sphenoids are present. There is no 
“ anterior clinoid cartilage,” and the “ posterior clinoid wall ” does not rise behind the 
small pituitary body (py.). 
Attached to the under face of the fore half of the free trabecular rod is an oval, bony 
plate with bristling teeth. This is the palatine bone ; and in front of it is a similar 
toothed plate, the vomer {ppg., v.). 
Near the frontal edge of the face, below, a very small pair of bony spicules are set 
transversely, each bearing two recurved teeth. These are the premaxillaries (pa;.). 
Between these and the vomer, but further outwards, the nasal sacs ( ol .) are seen ; 
they are becoming perforated. 
* These minute skulls have received most careful attention, in preparing them for both lateral and bird’s-eye 
views : this has been done by staining some of them with carmine, and by imbedding others for sections, and 
the colouring of such sections with picro-carmine, which gives a rose-pink colour to the cartilage, and makes 
the bony plates scarlet and the epidermis brown. This latter was my son’s work. 
