DEVELOPMENT OF THE SKULL IN THE MAMMALIA. 
169 
were a coronoid bone, like that of a Reptile, this postero-superior part of the dentary 
would be distinct. The inner alveolar plate corresponds to the splenial bone; an 
angulare, a supra-angulare, or an articulare would have to be sought for on the malleal 
part of the deep or primary mandible. 
The stapes (st.) was dislocated out of the fenestra ovalis in making this preparation. 
I did not figure the rest of the hyoid arch. 
Fourth Stage (continued).— A series of vertically-transverse sections of the head of an 
embryo Mole , f inch long from snout to root of tail. 
During the time that the embryo of the Mole grows from 15 mm. to 18 mm., the 
tissues gain much increase of solidity; this is, indeed, the best stage for studying the 
structure and relations of the chondrocranium, and the superficial bony plates which 
it draws to itself for increase of strength. 
The long snout (see Plate 25, figs. 2, 3) admits of being sectioned into a large 
number of slices ; I have, however, only figured a moderate percentage of those that 
were made in this case (Plate 23, figs. 16-19), which was an embryo of the same size 
as that which was dissected to show the chondrocranium (Plate 25, figs. 2 and 3). 
The 1st Section (Plate 23, fig. 16) is in front of the outer nostrils ; here the upper 
part of the section is the narrower ; the lower is the narrow part further back. Here 
the septum nasi ( s.n .) is perfect, and is dilated both above and below; the alre ( al.n.) 
are thick above, turn inwards below, and the thick growth below turns upwards. 
Part of the folded part below is so curled round as to appear in this section as a 
distinct segment. 
2nd Section (Plate 23, fig. 17). —This is close to the nostrils, and the nasal passages 
are seen projecting outwards, right and left. The septum nasi (s.n.) is thinnest at 
this part, and here the alse nasi (al.n.) turn inwards abruptly, whilst the folds that 
form the floor (nf) are reaching further upwards. 
3 rd Section (Plate 23, fig. 18). —Here the narial tubes are surrounded by cartilage, 
for the floor has reached the roof (nf, al.n.), and now the septum (s.n.) is very thick 
above, and of considerable thickness below the most dilated part. 
Ath Section (Plate 23, fig 19). —Here the roof and floor are confluent at the sides, 
and in this section it is seen that the floor has turned inwards so as to project, laterally, 
by its proper end, into the nasal passage, the fold uniting with the dilated septum 
(s.n.) —or intertrabecula—by its upper face. 
Here the mandibles are cut across in their fore part, and the basimandibular rod 
(b.mn.) unites the two Meckelian rods (mk.). 
5th Section (Plate 23, fig. 20). —-This is behind the snout, in the front part of the 
proper nasal labyrinth, which is now open below. Here the nasal septum is seen 
to be merely the round intertrabecula—like that of an embryo Bird—with the nasal 
roofs, however, united to it. The only remnant of the floor, so large and perfect in the 
MDOCOLXXXV. 
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