OF THE MARSIPOBRANCH FISHES. 
443 
nerve enters, are covered with a thin layer of true cartilage, which, however, like the 
rest, is soft , at present. 
The only skeletal part formed inside the head cavities and branchial arteries is the 
pedicle, with its hooked pterygoid rudiment ( pd., pg.) ; this state of things continues 
until the time of transformation. 
But the extra-branchial basket-work is one of the first parts to be developed, as the 
sections now to be described show ; the pattern of the basket-work is figured from a 
later stage (Plate 25, fig. 8), but it merely differs from that of these embryos in size 
and a somewhat increased density and strength of the cartilage. The sections now to 
be described will be better understood by reference to a figure of Balfour’s (vertical 
section of an embryo half the size of those from which these transverse sections were 
made) (see ‘Comp. Embryol.,’ p. 75, fig. 43). 
Vertically transverse sections of the head and branchial region of embryos of P. planeri, 
7‘8 mm. and 9 - 5 mm. in length. Magnified 150 and 300 diameters. 
ls£ Section (Plate 24, fig. 1, 150 diams. ; see also Plate 8, figs. 1-3).—This is through 
the fore brain (C 1 .) and upper lip ( u.lp .) of the larger embryo. Here the huge relative 
size of the brain is shown, filling the membrano-cranium. The parts which concern us at 
this part are the thin tilted ends of the trabeculae ( tr .), which are lanceolate in section, 
and are connected together by a definite tract of soft cells. In part of a similar 
section of the smaller embryo (Plates 24, fig. la, 300 diams.) we see that the cartilage 
is two cells thick in the middle, and that the intervening soft tract is in two layers 
near the cartilage, ready to form the trabecular commissure (see Plate 19, figs. 4, 5, tr.). 
2nd Section (Plate 24, fig. 2, 150 diams.).—This is still through the upper lip 
{u.lp.), the fore brain (C l .) is somewhat smaller at this part, the trabeculae {tr.) are now 
oval in section, and the special layer of indifferent cells, ready to become a transverse 
band of cartilage, is not seen. In a partial section of the lesser embryo at this part 
(Plate 24, fig. 2a, 300 diams.), the young cartilage (tr.) is three cells thick. 
3rd Section (Plate 24, fig. 3, 150 diams.).—This is through the fore brain (C 1 .) 
behind the upper lip, and therefore through the chin (see Plate 8, figs. 1-3); the 
tilted trabeculae (tr.) are broader than in the last section. 
4 th Section (Plate 24, fig. 4, 150 diams.).—In this section, through the hind part of 
the fore brain (C 1 .) and the pituitary region (py.), the trabeculae (tr.) are thicker and 
less tilted ; this is immediately in front of the Gasserian ganglion. 
5 th Section (Plate 24, fig. 5, 150 diams.).—This is through the mid brain (C 3 .) ; the 
fore part of the Gasserian ganglion is cut across, and also the stem of the 5th nerve (V.) 
as it passes over a tract of cartilage growing directly from the cartilage (iv.). The 
notochordal apex ( nc.) is cut across close behind the pituitary region. Another figure 
(Plate 25, fig. 1, 300 diams.) shows the basis-cranii and its facial outgrowths on a larger 
scale. The oval section of the cartilage (iv.) lies in a nearly horizontal position and 
