HEAD CAVITIES AND NERVES OF ELASMOBRANCHS, 77 
of the hinder cavities. Its more deeply situated portion is 
shown in figs. 26 and 27, at a stage between m and n; and figs, 
36 and 37 show that at stage o it not only persists but is con- 
siderably larger than at the earlier stages. 
The second or mandibular cavity presents during stages k and 
L, as shown in fig. 5 (2), and in figs. 7 to 12, a dorsal dilated 
portion, and (fig. 5) a laterally compressed ventral prolongation 
extending down the whole length of the mandibular arch. 
During these two stages the dorsal dilated end lies very close to 
the surface immediately beneath the epiblast ; it lies at a more 
superficial level than the cavities in front of and behind it, so 
that in a series of longitudinal and vertical sections the most 
superficial sections cut this cavity alone (fig. 7, 2); the subse- 
quent ones cut all three cavities (figs. 8 to 10); while still 
deeper ones cut the first and third only (fig. 12); and deeper 
still only the median portion of the first (fig. 13). 
By the commencement of stage m the mandibular cavity is 
still but little altered (vide figs. 21, 22, and 23, 2); itis rela- 
tively smaller, but still presents a dorsal dilatation and a flat- 
tened prolongation extending down the mandibular arch. By 
the middle or end of stage m the dorsal portion has atrophied 
and disappeared; the prolongation into the mandibular arch per- 
sists longer; its walls become converted into muscles,! but the 
distal portion of the cavity still persists at stage o (vide fig. 34). 
The third or hyoidean cavity is very similar to the second ; it 
is from the first rather smaller, and situated, as already noticed, 
at a rather deeper level. Its position and relations are shown at 
stages K and 1 in figs. 4, 5, and 8 to 12 (3). It is also shown 
between L and m in figs. 24 and 25 (3). Like the mandibular 
cavity it presents a dorsal dilated part and a ventral laterally 
compressed portion extending down the hyoidean arch. The 
dorsal portion, like the corresponding part of the mandibular 
cavity, disappears during stage m; the ventral part persists 
longer, and its terminal portion is shown at the extremity of the 
hyoidean arch at stage m im figs. 15 and 16. Its walls like 
those of the mandibular cavity become ultimately converted into 
muscles. 
Relations of Cranial Nerves to Head Cavities.—Certain 
of the cranial nerves have, as has already been shown by 
Balfour, very definite and very important relations to the head 
-cavities, and especially to the dilated dorsal ends of the three 
anterior cavities. A full account will be given later on under 
the headsof the seve ral nerves ; here I propose merely to note the 
general relations. 
1 Balfour, op. cit., p, 208. 
