138 NOTOCHORD. 



The changes by which this takes place can easily be followed 

 in longitudinal sections. In PL XL fig. 11 the migration of the 

 nuclei has commenced. They are still, however, more or less 

 aggregated at the centre, and very little protoplasm is present 

 at the edges of the notochord. The cells, though more or less 

 irregularly polygonal, are still somewhat flattened. In PL xi. 

 fio-. 12 the notochord has made a further progress. The nuclei 

 now mainly lie at the side of the notochord, where they exist in 

 a somewhat shrivelled state, though still invested by a layer of 

 protoplasm. 



A large portion of the protoplasm of the cord forms an 

 almost continuous layer in close contact with the sheath, 

 which is more distinctly visible in some cases than in others. 



While the changes above described are taking place the 

 notochord increases in size. At the age of fig. 11 it is in the 

 anterior part of the body of Pristiurus about O'll Mm. At the 

 age of fig. 12 it is in the same species 0*12 Mm., while in Scyl- 

 lium Stellare it reaches about 0*17 Mm. 



During stage K (PL x. fig. 8) the vacuolation of the cells of 

 the notochord becomes even more complete than during the 

 earlier stages, and in the central cells hardly any protoplasm 

 is present, though a starved nucleus surrounded by a little 

 protoplasm may be found in an occasional corner. 



The whole notochord becomes very delicate, and can with 

 great difficulty be conserved whole in transverse sections. 



The layer of protoplasm which appeared during the last 

 stage on the inner side of the cuticular membrane of the 

 notochord becomes during the present stage a far thicker and 

 more definite structure. It forms a continuous layer with 

 irregular prominences on its inner surface ; and contains nume- 

 rous nuclei. The layer sometimes presents in transverse sec- 

 tions hardly any indication of a division into a number of 

 separate cells, but in longitudinal sections this is generally very 

 obvious. The cells are directed very obliquely forwards, and 

 consist of an oblong nucleus invested by protoplasm. The 

 layer formed by them is very delicate and very easily destroyed. 

 In one example its thickness varied from '004 to '006 Mm., in 

 another it reached '012 Mm. The thickness of the cuticular 

 membrane is about '002 Mm. or rather less. 



