5 
412 MR. W. K. PARKER OK THE SKELETOH 
A. — The skeleton of the Lamprey after transformation. 
I shall at once take up the permanent form of the skeleton, that it may link itself 
on to that of the Myxinoid form described in the First Part. 
a. Dissections of the young ( one-tliird grown ) of Petromyzon marinus. 
The substance of which the skeleton of the Lamprey is composed is mainly the two 
kinds of true cartilage, soft and hard. The latter is the densest kind of hyaline 
cartilage seen in the Vertebrata ; greenish, when looked at by reflected light, it is of 
a clear amber colour when seen in the mounted transparent sections ; in the early 
embryo both kinds are similar, having very little intercellular substance. The 
vacuolar tissue is in the Lamprey confined to the notochord, the sheath of which, as 
in the Myxinoids, is a strong web of white fibrous tissue. 
1. The post-cephalic part of the axial skeleton. 
Here we have, after metamorphosis, roughly conical patches of hard cartilage 
(Plate 18, fig. 1; and Plate 22, figs. 7-9, n.a.) ascending the sides of the theca verte- 
hralis, mounting above it, and having their bases attached to the side of the sheath of 
the notochord, which maintains its size and its cylindrical form between these paired 
rudiments, the beginnings of neural arches. This is almost the only part of the post- 
cephalic region in which anything but fibrous tissue is found as a skeletal structure, 
for the huge pharnyx is supplied with, and dominated by, a cranial nerve, right and 
left, the vagus. Yet cartilage is continued backwards, as the pharyngeal and 
pericardial skeleton, as far as the 13th pair of vertebral arches (Plate 18, fig. 1 , ex.hr., 
pcd.c., ua 13 .). 
2. The craniofacial apparatus — cranium, proper, and ventral or intra-visceral 
outgroivths [rudimentary arches ). 
This continuous growth of hard cartilage (mainly) has a much less antero-posterior 
extent than in the Myxinoids, only part of the hyoid, and none of the intra-hranchial 
bars, being developed inside the temporary embryonic head-cavities and persistent 
aortic arches. 
Nevertheless, such evident rudiments of visceral arches as can be found, albeit 
merely seen as basi-cranial “ outgrowths ” growing downwards, will be spoken of in 
familiar terms, the adult Myxinoids, larval Anura, and the larva of Lepidosteus, giving 
us some boldness in the determination of these ventral growths of the head. 
For in all these types we have a more or less continuous cartilaginous growth of the 
