770 PROFESSOR W. C. M‘INTOSH AND MR E. E. PRINCE ON 
They are not, however, regularly arranged, and the distal enlargement protruding from 
the integument is often absent. Thus, in the gurnard above referred to, nerve-filaments 
were observed passing across the sub-epidermal space from the trunk, and terminating in 
the skin without an enlarged sensory-organ. The external sensory-organ (Pl. VI. figs. 
8, 8a) consists of a somewhat elliptical aggregation of granular columnar cells, from which 
a number of very fine and apparently rigidly erect palpocils (plp) project. A delicate 
nerve-filament (nv) passes from it to the muscular plates (my), and so to the central 
nervous system. This filament shows a slight enlargement at its proximal end, and 
another dilatation just as it approaches the external sensory-organ. 
As noted on a previous page, large spaces (ss) filled with a clear plasma exist below 
the integument (ep), separating it widely from the trunk (Pl. XV. fig. 7), and across 
these spaces in more advanced embryos fine nervous threads pass from the myotomes to 
the skin, occasionally giving off in their course delicate secondary filaments. The nerve 
going to the cephalic sensory-organ apparently comes from a cutaneous sensory branch; 
and Horrman states (No. 69) that the development of the ramus lateralis nervi vagi 
always precedes the appearance of these sensory-organs. 
No sections of the early stages show the longitudinal sensory tract called the “ lateral 
line” in fishes. There is, however, in the caudal trunk of an advanced haddock a canal 
apparently surrounded by nervous cells and mucous tissue which stains deeply (Pl. XI. 
fig. 16), but it can only be traced a short distance in the tail of the example referred to. 
As noticed elsewhere, the facial region is provided with numerous papilliform sensory 
bodies, and these are large and very noticeable in what may be called the maxillary or 
sub-prosencephalic region. They exhibit a structure similar to the lateral sensory-organs, 
and are composed of lengthened spindle-shaped cells (Pl. XXI. fig. 7, sb). 
Alimentary Canal.—In its earliest condition the alimentary tract consists merely of 
a thickened sub-embryonic layer of hypoblast, intervening between the neurochord above 
and the yolk, or rather periblastic cortex of the yolk, below. Posteriorly, when little 
more than one-third of the yolk is covered by the blastoderm, the hypoblastic cells 
beneath the embryonic axis, as already pointed out, assume a distinct columnar character 
(hy); a lumen (hg) appears below, which is arched over by columnar hypoblast, and 
has a floor of nucleated periblast (per, Pl. IV. figs. 5b, 6). This is the first indication of 
the alimentary tube, and it forms the posterior section—the continuity of which with the 
neurenteric canal and medullary groove has been already described. From the arched 
enteric roof the notochord is differentiated. The lumen at first extends but a very 
short distance forward, and is lost in an anterior aggregation of hypoblastic cells. These 
cells, formed by the proliferation of the thin sheet of invaginated hypoblast, reach as 
far as the cardiac region, where they thin out rapidly, and form a delicate limiting 
membrane below the head (hy, Pl. IV. figs. 3, 4). As this thickened mesenteric mass 
arises, the embryo is necessarily raised from the yolk except in the cephalic region— 
the snout still lying in close contact with the yolk below, so that a pseudo-cranial 
flexure is produced, and a pericardial space (pd) formed beneath the otocystic region 
