430 
MR. W. K. PARKER ON THE SKELETON 
first pair of neural arches of the spine (n.a. 1 ) are cut across, some of the notochord, and 
part of the first pair of branchial pouches. 
In the next section, taken at a lower plane (Plate 1G, fig. 2) the lower part of the 
eye-halls is shown, and the oral cavity is exposed with the intervelar fringed shelf 
( i.v.s .), the two-leaved “vela ( vl .),” and the fore part of the branchial bronchus ( br.c .) 
The first (arrested) cleft, and the first proper branchial opening are hidden under the 
folds of the “ velum,” but the 2nd and 3rd branchial apertures are shown clearly, 
and also the cavities of the three foremost pairs of pouches ( br.p.) Both the cerato- 
hyal (c.hy.) and the stem of the intervelar skeleton (i.v.s.) are cut across. In front, 
the annulus (l.l\) is cut across, right and left, and also the labial distal mandibular 
cartilages ( l.d.m .). Then close to the floor of the mouth the highest part of the basi- 
liyal ( b.hy.) is caught; outside it, the lower edge of each 2nd lateral upper labial (u.l 3 .); 
and close under the eye-ball the pterygo-palatine bar (pp-<jf [In this figure for 
i.b.ct 1 . read i.b.a 2 ., and for br.p 1 . read 6r.p 2 .] 
Vertically-transverse sections of the head of the adult Petromyzon fluviatilis. 
In one rather small specimen nearly six scores of thin sections were made, stained 
with eosin , and mounted as transparent objects, in Canada balsam ; of these rather more 
than one in four were drawn (Plates 20-22 ; and Plate 23, fig. 4). These reached from 
the sucking disk to the 2nd branchial pouch. Other sections were made by hand 
(solid) from the hind part of the branchial region of a very large specimen, which had 
been solidified in a solution of chromic acid (Plate 23, figs. 5-7). In these latter sec¬ 
tions the muscular masses are more accurately shown than in the thin specimens that 
have been interpenetrated with oils and resin. These parts, however, are drawn in the 
figures merely to indicate the relation of the skeletal parts ; it does not enter into my 
plan to describe them. As they appear in the figures they will be easily understood 
by the Anatomist. It is utterly impossible for any figures to give an adequate idea of 
the beauty of the transparent sections.'" 
l6'f Section (Plate 20, fig. l).—This section is through the fore part of the suctorial 
disk,t and this, and the next, are in front of the trabecular cornu. The median upper 
labial (u.l 1 .) is cut across in its narrower fore part, and beneath it the annular labial — 
the azygous element of the lower series (l.l 1 .)—is seen cut across right and left of the 
high arched space shown by section of the great disk. This is near the front part of the 
annular cartilage ; it is highest and thinnest at this part, and is bent upon itself below the 
middle. Hence in the outer view (Plate 18, fig. 1, l.l 1 .) the upper part of the side seems to 
be bevelled. Below this, at a small distance, there is a somewhat lesser section of car¬ 
tilage, which is not curved ; this has suctorial teeth on its inside, and it lies opposite the 
* The thin sections were made by one of my sons, and the camera-drawings by another, 
f To follow the meaning of the parts displayed by these sections it is necessary to keep the form of the 
head of the undissected Lamprey before the eye (Plate 8, figs. 11-13) and also the structure of the cranio¬ 
facial apparatus as displayed by the dissections and solid sections (Plates 18 and 23). 
