DEYELOPMEXT OF ELASMOBRANX'H FISHES. 155 



to P further changes take place in the notochord : the remains 

 of the cells become more scanty and the intercellular tissue 

 assumes a radiating arrangement, giving to sections of the noto- 

 chord the appearance of a number of lines radiating from the 

 centre to the periphery (PL xii. fig. 11). 



The sheath of the notochord at first grows in thickness, and 

 during stage L there is no difficulty in seeing in it the fine radial 

 markings already noticed by Mliller^ and Gegenbaur^ and re- 

 garded by them as indicating pores. Closely investing the sheath 

 of the notochord there is to be seen a distinct membrane, which, 

 though as a rule closely adherent to the sheath, in some 

 examples separates itself from it. It is perhaps the membrane 

 identified by W. Miiller^ (though not by Gegenbaur) as Kol- 

 liker's ' membrana elastica interna.' After the formation of 

 the cartilaginous investment of the notochord, this membrane 

 becomes more difficult to see than in the earlier stage, though 

 I still fancy that I have been able to detect it. The sheath 

 of notochord also appears to me to become thinner, and its 

 radial striation is certainly less easy to detect ^ 



1 Jenaische Zeitschrift, Vol. vi. ^ ^ioc. cit. ^ Loc. cit. 



^ Gegenbaur makes the reserve statement with reference to the sheath of 

 the notochord. For my own sections the statement in the text^ certainly holds 

 good. Fortunately the point is one of no importance. 



11—2 



