It is a very striking fact that the brain of Ammocoetes is much too small for the brain-case, and that the space between brain and brain-case is filled up with a very peculiar glandular-looking tissue, which is found in Ammocoetes and not elsewhere. Further, it is also striking that in the brain of Ammocoetes there should still exist the remains of a tube extending from the IVth ventricle to the surface at the _conus post-commissuralis_, which can actually be traced right into this tissue on the outside of the brain (see Fig. 13, _a-e_, Pl. XXVI., in my paper in the _Quarterly Journal of Microscopical Science_). {210}This, in my opinion, is the last remnant of one of the old liver-ducts which extended from the original stomach and intestine into the cephalic liver-mass. This glandular-looking material is shown surrounding the pineal eye and its nerve, in Fig. 31, also in Fig. 22, and separately in Fig. 92. It is composed of large cells, with a badly staining nucleus, closely packed together with lines of pigment here and there between the cells; this pigment is especially congregated at the spot where the so-called liver-duct loses itself in this tissue. The protoplasm in these large cells does not stain well, and with osmic acid gives no sign of fat, so that Ahlborn's description of this tissue as a peculiar arachnoideal fat-tissue is not true; peculiar it certainly is, but fatty it is not. [Illustration: FIG. 92.--DRAWING OF THE TISSUE WHICH SURROUNDS THE BRAIN OF AMMOCOETES.] This tissue has been largely described as a peculiar kind of connective tissue, which is there as packing material, for the purpose of steadying a brain too small for its case. On the face of it such an explanation is unscientific; certainly for all those who really believe in evolution, it is out of the question to suppose that a brain-case has been laid down in the first instance too large for the brain, in order to provide room for a subsequent increase of brain; just as it is out of the question to suppose that the nervous system was laid down originally as an epithelial tube in order to provide for the further development of the nervous system by the conversion of more and more of that tube into nervous matter. Yet this latter proposition has been seriously put forward by professed believers in evolution and in natural selection.