DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 
55 
for articulation with the incus, and in the tendon of origin of the stapedius muscle 
(. st.m .) there is a round “ interhyal ” (infrastapedial) nucleus of cartilage ( i.hy .). 
Besides the superficial dentary bone (d.), with its splenial and coronoid tract, there 
is only one other osseous centre shown in this figure ; that is the first rudiment of 
the malleus (ml.) —as a bone—and it is the liomologue of the “articulare externum” 
of the Ovipara. 
Skull of an embryo of Bradypus (Arctopithecus)-? sp. (5 inches long). — Second Stage. 
We must in these comparisons keep in view the fact that we are dealing with 
modifications of growth in the individual, and also with specific, and even generic 
differences ; these latter are not very great, and can easily be allowed for. 
This skull of an embryo, almost twice as advanced as the last, looks more embryonic 
than it, for the face is shorter, and the cranium swells up above, giving it an almost 
Simian character (Plate 8, figs. 5 and 6). 
The lower view (Plate 8, fig. 5) is curiously like, and yet in several things unlike, 
that of the earlier embryo of the Unau (Plate 8, fig. 1). The nostrils and alinasal 
region (e.n., cd.n.) are quite similar in both, but behind them the premaxillaries (p: r.) 
are smaller, but have longer palatine processes. The anterior palatine foramina, 
exposing the openings of Jacobson’s organs (j.o.), are better enclosed as two lanceo¬ 
late spaces, for the maxillaries (mx.) have closed in, in front, and bind on both the 
palatine processes behind, and the feeble dentary tract of the premaxillaries. 
The hard palate is not a squared oblong tract here, but is elegantly urceolate, for 
the fore half forms an outline of more than half a regular ellipse* then it narrows 
gently, and then widens again, to grow out into handles or horns, where the palatine 
bones (pa.) take their part in its formation. 
The whole maxillary region is well flattened out; a pair of middle palatine foramina 
end the submedian grooves, which bend outwards near their fore end. One-fourth 
further back the palatine plates are cut away, in a semi-oval manner, to make room for 
two wedges of bone, the fore part of the palatines. The rounded end of the alveolar 
tract of each maxillary rests against the thick outer part of the palatine, and the 
sockets, with their teeth, scarcely enlarge from the fifth, forwards, to the third ; the 
second is the largest tooth, and swells the bone out there to its greatest convexity. 
The first tooth is less than a fourth the sixe of the second, and lies midway between 
it and the front convergence of the bone. 
Outside the second and third teeth the maxillary enlarges outwards, so as to hide 
the infraorbital foramen in this aspect; this squarish outgrowth gives a horned 
appearance to this part of the skull, so viewed. The hind part of this tract articu¬ 
lates with the jugal (j.), being rugged where that bone fits in, and then there is a 
rounded notch between this short jugal process, and the outer alveolar wall. Each 
large jugal (y.) stretches outwards and backwards to its free but notched end; the 
