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 (pip) 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 Hoffman 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 XL 

 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. 56, 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 



