OF THE MARSIPOBRANCH FISHES. 
439 
apparatus. Here the upper extra-branchial bands (ex.br.) are cut across; the 
uniting bands above and under the aperture; and the median inferior junction of the 
arches. 
31 st Section (Flate 23, fig. 4).—This is the last of the thin sections figured, and is 
from the beginning of the second third of the branchial region; both the external 
apertures (e.b.a.) are laid open. The neural arches (n.ct.) are not so wide apart 
above, and they have crept some distance down the side of the notochord ; under these 
hard tracts the soft upper bands of the basket-work (ex.br.) are cut across, then two 
of the lesser bars, then a main bar, bending inwards behind its own aperture (see 
Plate 18, fig. 1), and then a common inferior band at a very narrow part. The 
passages between the pouches are the great artery (ao.), the pharynx (phx.), and the 
branchial canal ( br.c .); the median basi-hyobranchial bar is not seen, but the hind part 
of its huge muscular lingual apparatus is shown. 
32 nd Section (Plate 23, fig. 5).—This is the first of the transversely-vertical solid 
sections, made a little obliquely so as to show the external branchial aperture, the last, 
on one side (only) with a few of the folds (br.p'.). The small flat myelon {my.) is seen 
in the canal formed by the theca vertebralis ( th.v .), and under it the notochord ( nc .) 
is at its largest size. The heart (h.) is cut through in its fore-part, with the basket- 
work of the pericardial region (pccl.c.) cut across. The pharynx (p>hx.), at the end of 
the branchial canal, is larger, but is pushed out of the mid-line by the heart. This 
section is between two pairs of neural arches. 
33rd Section (Plate 23, fig. G).—The second of these solid sections is through the 
middle of the heart (h.), and shows the extra pericardial (pccl.c.), as an almost 
perfect girdle ; here the opposite face of the section has been drawn, so that the 
pharynx (plix.) is seen on the other side ; the tips of two neural arches (n.a.) have 
been cut through. 
2>ith Section (Plate 23, fig. 7).—Here we see the fundus of the tilted bowl, the 
pericardial cartilage (pcd.c.) ; the pharynx is in the figure on the same side as in 
fig. 5, the same face of the section being drawn; only the tips of a pair of neural 
arches are cut across. The muscular masses in these three figures are shown in their 
full (uncontracted) bulk.'"' 
On the skeleton of the embryo of Petromyzon planeri, 7'8 mm. to 9'5 mm. in length. 
These small, delicate, worm-like embryos, the larger of which only, namely, those about 
l>-inch long, were worked out by me, came to me through Prolessor F. M. Balfour, 
who was enabled to breed them (by artificial impregnation) through the kindness of 
Osbert Salvin, Esq., F.B.S. The sections were lent me by Professor Balfour, and 
what I have to show with regard to them may be added to his excellent account of 
* Schneider (plate 10, fig. 1) shows the cartilages of the tail-fin: these parts do not enter into my 
plan. 
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