144 THE LATERAL LINE. 



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

 those of the posterior, commence to assume a tubular arrange- 

 ment, 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 con- 

 verted into a canal it recedes from the surface. 



In stage P the first indication of segmental apertures to the 

 exterior make their appearance, vide PL Xll. fig. 4. The lateral 

 line forms a canal situated completely below the skin, but 

 at intervals (corresponding with segments) sends upwards and 

 outwards prolongations towards the exterior. These prolonga- 

 tions do not during stage P acquire external openings. As 

 is shown 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 tlie lateral line ; they are products of the mucous layer of 

 the epidermis. They eventually form either canals with nume- 

 rous openings to the exterior, or isolated tubes with terminal 

 ampulliform dilatations. 



I have not definitely determined whether the canal-system 

 of the head arises in connection with the lateral line, or only 



