84 GENERAL SKETCH OF 



which emerge close to the surface of the medulla, just below the 

 overlapping posterior part of the optic lobe. 



The vagus or tenth arises by two roots, the first, which 

 probably includes the fibres of the ninth, issuing from a point 

 near the lateral summit of the medulla oblongata. It per- 

 forates the auditory cartilage and sends branches to the gill- 

 arches and pharynx. The second part describes a curve and 

 can be traced to the median region of the medulla below the 

 floor of the fourth ventricle, and above the pyramids, part of its 

 fibres having a more superficial origin. They form in front of 

 the pectoral girdle a large double ganglion below and, to some 

 extent, internal to the ear. Fibres pass, further, to the pharynx 

 and branchial arches, and, from a smaller ganglion, pharj^igeal 

 and cardiac branches are given off. 



Lateral sense-organs. 



Generally one of these occurs on the top of the head just 

 behind the eye, a second behind the pectoral fin, and one or two 

 along the caudal region. Each consists of a somewhat elliptical 

 aggregation of granular columnar cells from which a number of 

 very fine palpocils project. A delicate nerve-filament passes from 

 the apparatus to the muscular plates, and this shows a slight 

 enlargement at its proximal end, and another as it approaches 

 the sensory organ. In some instances, the latter is absent 

 whilst the nerve is present. In the haddock, the facial region 

 has numerous papillary sensory bodies, which have a similar 

 structure, viz. lengthened spindle-shaped cells. In the gurnard 

 also, on the snout, tubes with cushions of columnar epithelium 

 and sensory hairs occur, and they communicate with the 

 cushions. 



Skin. 



It has already been mentioned that in early embryonic 

 life there is externally a flattened layer or corneous stratum 

 .of epiblast, and beneath it is the nervous layer. Soon after the 

 notochord is defined, these layers extend as a distinct integu- 

 ment all over the embryo and its yolk-sac. The inner stratum 

 at a late stage consists of several layers of rounded cells, the 



