THE LATERAL LINE. 365 



the formation of a cavity in the midst of the cells of the lateral 

 line. Its walls are formed by a layer of columnar cells on the 

 inner side, and flattened cells on the outer side, both layers 

 however appearing to be derived from the mucous layer of the 

 epidermis. The outer layer of cells attains its greatest thickness 

 dorsally. 



During stages M, N, O, the lateral nerve gradually passes 

 inwards into the connective tissue between the dorso-lateral and 

 the ventro-lateral muscles, and becomes even before the close of 

 stage N completely isolated from the lateral line. 



The growth of the lateral line itself remains for some time 

 almost stationary; anteriorly the cells retain the gable-like 

 arrangement which characterised them at an earlier period, but 

 cease to enclose the nerve; posteriorly the line retains its original 

 more complicated constitution as a closed canal. In stage O 

 the cells of the anterior part of the line, as well as those of the 

 posterior, commence to assume a tubular arrangement, and the 

 lateral line takes the form of a canal. The tubular form is due 

 to a hollowing out of the lateral line itself and a rearrangement 

 of its cells. As the lateral line becomes converted into a canal 

 it recedes from the surface. 



In stage P the first indication of segmental apertures to the 

 exterior make their appearance, vide PI. 13, fig. 4. The lateral 

 line forms a canal situated completely below the skin, but at 

 intervals (corresponding with segments) sends upwards and out- 

 wards prolongations towards the exterior. These prolongations 

 do not during stage P acquire external openings. As is shewn 

 in my figure, a special area of the inner border of the canal of 

 the lateral line becomes distinguished by its structure from the 

 remainder. 



No account of the lateral line would be complete without 

 some allusion to the similar sensory structures which have such 

 a wide distribution on the heads of Elasmobranchs ; and this is 

 especially important in the present instance, owing to the light 

 thrown by a study of their development on the origin of the 

 nerves which supply the sense-organs of this class. The so- 

 called mucous canals of the head originate in the same way as 

 does the lateral line; they are products of the mucous layer of 

 the epidermis. They eventually form either canals with nume- 



