DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 
47 
Skull of embryo of Cholopus didactylus (3^ inches long).—First Stage. 
In this type the nostrils, as seen in the lower vieiv (Plate 8, fig. 1, e.n), are not 
inferior, but lateral; and the alinasal floor ( al.n.) shows but a small distance in front 
of the premaxillaries (px.) ; the basal alse reach to these openings, as the roofs do, 
above (fig. 2, al.n.). The premaxillaries are as small as in Tatusia, but they have each 
a well-formed palatine process, nearly as large as the narrow, anterior, or dentary 
tract. The pointed ends of these submesial spurs do not reach the inner part of the 
maxillary palatine plates (mx.), for these retreat considerably, leaving a considerable 
angle of membrane exposed ; this, and the two round notches between the inner and 
marginal parts of the premaxillaries, form a cordiform tract, in the anterior lobes ol 
which we see the openings of Jacobson’s organs (j.of In this early embryo, 
about one-tliird ripe, the palatine plates of the maxillaries (mx.) are only just 
approximating even in the middle ; in the hind part, as in front, there is a considerable 
angular space, enclosed by these ingrowths of bone. The whole of this double tract 
is nearly square; the breadth across the foremost alveoli is greater than the length 
in the middle ; at the sides, the last pair of alveoli overlap the palatines (pa.), and 
there make the side measurement the longer of the two. Already the foremost 
alveoli, with their teeth, project most, whilst the last pair project least; there is room 
for two sockets, with their teeth; between the first and second, a shallow groove is seen 
along this edentulous space. The inter-alveolar flange is separated from the inner 
part of the bone by a less distinct groove than that seen in Tatusia; behind, it forms, 
at its palatine end, a definite and widish space. In this view, the facial part of the 
maxillaries is seen, as a right and left wing ; in this space the infraorbital foramen (V 3 .), 
is well shown; its bridge, or floor, is narrow. The rest of the palate is very peculiar, 
it is quite a rough, generalized structure, the palatines (pa.) themselves being not so 
much developed in this region as the pterygoids are in Tatusia hybrida, at a somewhat 
later stage. There is an angular space between the palatine plates of these bones, 
in front, and behind ; they do not meet at the mid line. They then, like the pterygoids 
of Tatusia hybrida, diverge, so as to leave a semi-oval space, in which the broad 
basis cranii is seen, or all its presphenoidal region (p.s.). The ascending or cranial part 
of the palatines is very limited; the wall formed by the bone is extremely thick and 
rough, and behind the last tooth socket each bone forms a rounded boss, from which 
the rest of the bone diverges gently. 
But all this roughness is seen in a double degree in the pterygoids (pg-), which 
have a very reptilian simplicity of form, and independence of the basis cranii, to which 
their attachment is very limited. Each bone is like a rough nut, in miniature ; it is 
somewhat scooped postero-laterally, and at its end has a cupped cavity, filled with 
hyaline cartilage; the remnant of the Ichthyopsidan cartilaginous “upper jaw.” 
The whole palate is very generalized, and therefore very instructive ; it improves 
afterwards (see Plate 9, fig. 1), and yet in the large young of this genus the palatine 
