DEVELOPMENT AND LIFE-HISTORIES OF TELEOSTEAN FISHES. 753 



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 distinct 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. 21 ; 

 PL XXIII. figs. 1, 2, 3a; 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 (hy), it now rests 

 upon a floor of mesoblast (PL 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 XL 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 XL 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 trabecule, 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 trabeculae is noteworthy when connected with the early development of the neural 

 arches in the trunk. Parker's view that the trabeculae 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). 6 D 



