356 DEVELOPMENT OF ELASMOBRANCH FISHES. 



By the close of stage I the cuticular sheath of the notochord 

 has greatly increased in thickness. 



During the period intermediate between stages I and K the 

 notochord undergoes considerable transformations. Its cells 

 cease to be flattened, and become irregularly polygonal, and 

 appear but slightly more compressed in longitudinal sections 

 than in transverse ones. The vacuolation of the cells proceeds 

 rapidly, and there is left in each cell only a very thin layer of 

 protoplasm around the nucleus. Each cell, as in the earlier 

 stages, is bounded by a membrane-like wall. 



Accompanying these general changes special alterations 

 take place in the distribution of the nuclei and the protoplasm. 

 The nuclei, accompanied by protoplasm, gradually leave the 

 centre and migrate towards the periphery of the notochord. At 

 the same time the protoplasm of the cells forms a special layer 

 in contact with the investing sheath. 



The changes by which this takes place can easily be followed 

 in longitudinal sections. In PI. 12, 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 PI. 12, 

 fig. 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. n it is in the 

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

 age of fig. 12 it is in the same species O'I2 Mm., while in Scyl- 

 lium stellare it reaches about O'l/ Mm. 



During stage K (PL n, 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 pro- 

 toplasm may be found in an occasional corner. 



