i 
440 MR. W. K. PARKER ON THE SKELETON 
the development of this type in the 2nd vol. of his ‘ Comparative Embryology ’ 
(pp. 68-83, figs. 37-49). 
My figures are from one of the larger of these embryos (Plate 8, figs. 1-3) ; in the 
full-figure, the side view, about eighty myotomes are shown ; its great approach, in 
form, to the proper larva (or Ammoccete ) is seen by comparing the next figures (4-6) 
in the same plate. 
Dissection of the skull of an embryo of Petromyzon planeri, 7‘8 mm. long. 
As this is the lowest kind, of primordial skull worked out at present by me, I shall 
compare it with that of (A) the Ray;'"' (B) the Sturgeon; (C) the Lepidosteus; (D) 
the Salmon ; (E) the Axolotl; and (F) the Larval Frog and Toad. 
The large notochord (Plate 25, fig. 7, nc.) is almost straight; in an earlier stage 
(Balfour’s ‘Embryology,’ pp. 74, 75, figs. 42, 43) it is turned downwards at its fore 
end, and this bend is retained throughout life (Plate 23, fig. 1); it now reaches 
further forwards, relatively, than afterwards, and gradually lessens to a blunt point. 
Further than the hind margin of the pituitary space ( py .) it does not go. Taking the 
hind margin of the auditory capsules (cm.) as our place of measurement, behind, and 
the fore end of the trabeculae (tr.) as another, in front, the notochord reaches exactly 
half way. 
Measured thus some time after transformation (Plate 18, fig. 4, nc., tr.) the notochord 
is about two-fifths the length of the basis cranii; thus we see that it remains as an 
important basi-cranial structure, but in its earliest development it shows nothing of 
the cephalo-chordal character of Amphioxus. The membranous sheath of the noto¬ 
chord is very thick at this early stage ; at no stage have I found true cartilage in it, 
such as we see in the Elastnobranchs. The primary basi-cranial cartilages (iv., tr.) are 
para- chordal for their hinder three-fifths, and pro-chordal for their front two-fifths, but 
the front part is thick and wide, whilst the hind part attenuates gently to a fine, pointed 
end ; these two ends are, however, far back behind the ear-capsules; two-fifths of the 
parachordal length of the cartilage is behind the ear. Considering the size of the 
region supplied by the branches of the vagus nerve, it is not unreasonable to take all 
the parachordal region of the notochord as properly basi-cranial; the fact is, the head 
and spine are not divided off sharjfiy—there is no joint; afterwards, when the neural 
arches appear in the transformed Fish, the first pair of cartilages give us the beginning 
of the spine. 
These parts cling close to the sides of the notochord, so as to be crescentic hi section 
(see Plate 25, figs. 3, 4, iv., nc.) nearly to the middle of the auditory capsules (an.) : 
thence they diverge further and further from the axial rod. In front of that rod, in 
their pro-chordal part, the bars (trabeculae) are separated by a tract thrice their greatest 
* (A) Trans. Zool. Soc., vol. x., plate 35; (B) Phil. Trans., 1882, Plate 14; (C) Phil. Trans., 1882, 
Plate 30; (D) Phil. Trans., 1873, Plate 2; (E) Phil. Trans., 1877, Plate 22; (E) Phil. Trans., 1876, 
Plates 55, 56, and 60; and 1881, Part I. 
