3/6 DEVELOPMENT OF ELASMOBRANCH FISHES. 



fused, and are probably rightly regarded by Gegenbaur and 

 Gotte as representing a sparse intercellular matter. The external 

 protoplasmic layer of the notochord ceases shortly after stage K 

 to exhibit any traces of a division into separate cells, but forms 

 a continuous layer with irregular prominences and numerous 

 nuclei (PI. 13, fig. 9). In the stages subsequent to P further 

 changes take place in the notochord : the remains of the cells 

 become more scanty and the intercellular tissue assumes 1 a 

 radiating arrangement, giving to sections of the notochord the 

 appearance of a number of lines radiating from the centre to the 

 periphery (PI. 13, fig. n). 



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 Miiller 1 and Gegenbaur 2 , 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 3 (though not by Gegenbaur) as Kolliker's " mem- 

 brana 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 4 . 



EXPLANATION OF PLATE 13. 



COMPLETE LIST OF REFERENCE LETTERS. 



al. Alimentary tract, ao. Aorta, c. Connective tissue, ca v. Cardinal vein. 

 ch. Notochord. ep. Epiblast ha. Haemal arch. /. Liver. //. Lateral line, m c. 

 Mucous canal of the head. mel. Membrana elastica externa. nip. Muscle-plate. 

 mp'. Muscles of muscle-plate, na. Neural arch. nl. Nervus lateralis. rp. Rib 

 process, s d. Segmental duct. sh. Sheath of notochord. spc. Spinal cord, sp g. 

 Spinal ganglion, syg. Sympathetic ganglion, um. Ductus choledochus. v. Blood- 



1 Jenaische Zeitschrift, Vol. VI. 2 Loc. cit. * Loc. cit. 



4 Gegenbaur makes the reserve statement with reference to the sheath of the noto- 

 chord. For my own sections the statement in the text certainly holds good. For- 

 tunately the point is one of no importance. 



