eo 
DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 75 
is directly in contact with the inner surface of the epiblast. Later, however, this meso- 
blastic tissue extends and finds its way into the lateral sinuosities of the brain-surface, 
and it passes upward as a thin membrane composed of much flattened cells, which 
finally more or less completely invests the brain. The relation of the two is very 
intimate, and probably the pia mater is at this time separated, though any differentiation 
into distinet strata cannot be made out in the membranous investment (mes, Pl. IV. 
figs. 14,21). From this layer, however, the three membranes—dura mater, arachnoid, and 
pia mater—are ultimately differentiated. On its inner surface pigment rapidly develops, 
as early, indeed, as the fourth day after fertilisation in some forms (p, Pl. IV. fig. 13). 
We have thus a double covering over the brain, for to the simple ectodermal layer (ep, 
fig. 14), which primarily covers the neurula, there is added a thickened mesoblastic 
membrane (mes), constituting the primitive membranous cranium (Pl. IV. fig. 215 
Pl. XXIII. figs. 1, 2, 83a; Pl. XXIV. figs. 3, 5, 6). Meanwhile changes are proceeding 
at the base of the brain, and whereas it at first lay almost directly upon the yolk 
(Pl. IV. figs. 3, 4), separated only by a thin layer of hypoblast (iy), it now rests 
upon a floor of mesoblast (PI. IV. fig. 21). This mesoblast is apparently an exten- 
sion forward of the pectoral mesoblast, which pushes anteriorly as the notochord advances, 
and when the latter finally terminates beneath the mid-brain, a plate of intruding meso- 
blast is seen extending upon each side of it and passing as a thin sheet beneath the fore- 
brain. 
At the fore end of the notochord quite a dense plate is formed (Pl. XI. fig. 2), and 
a thickened continuation of this mesoblast passes beneath the eyes, forming a projecting 
ridge of epiblast with a core of mesoblast (Pl. XI. figs. 2, 3), which is doubtless 
Parker's “ sub-ocular band” (No. 117, p. 119). These two ridges form on each side a 
lateral flap or curtain, and the head is thus raised slightly from proximity to the yolk. 
As already pointed out, before an actual oral slit appears, an oral cavity exists whose 
roof slopes considerably on each side, and meeting in the middle line forms a highly 
arched chamber. A section through this acutely angular cavity in the region of 
the posterior prosencephalon (thalamencephalon) shows the apex, so to speak, marked 
by a small and solid mass of cells, a cylindrical rod in fact, which in sections further 
forward is found to flatten out in the form of a bilobed plate, strongly suggesting the 
union and depression of two cylinders of cells. We see then at this early stage, about 
the time of hatching, that the base of the brain is strengthened by two parachordal masses, 
which lie on each side of the notochord at its oral end, e.g., in the section of Anarrhichas 
(Pl. XXIV. fig. 3), and form a dense basilar plate, while further forward the flattened 
parachordals cease, and in their place two thin cylinders, the trabeculae, can be dis- 
tinguished (Pl. XXIV. figs. 5, 6), which, as just pointed out, unite and form beneath the 
thalamencephalon a single rod.* This rod again expands beneath and in front of the 

* The early appearance of the trabeculs is noteworthy when connected with the early development of the neural 
arches in the trank. Parker’s view that the trabeculw are ventral arches of the vertebral column, serially followed 
behind by the branchial arches, has been much disputed, and the early appearance of the neural arches of the vertebral 
column is opposed to PARKER'S view. 
VOL. XXXV. PART III. (NO. 19). 6D 
