DEVELOPMENT OF ELASMOBRANCH FISHES. 187 



indurated, so that they may be said to be invested by a mem- 

 brane^; the cells themselves have a flattened form, and each ex- 

 tends from the edge to the centre of the notochord, the long axis 

 of each being rather greater than half the diameter of the cord. 



The nuclei of the notochord are elliptical vesicles, consisting 

 of a membrane filled with granular contents, amongst which is 

 situated a distinct nucleolus. They stain deeply with h^ema- 

 toxylin. Their long diameter in Scyllium is about 0-02 Mm. 



The diameter of the whole notochord in Pristiurus durino- 



o 



stage I is about 01 Mm. in the region of the back, and about 

 0'08 Mm. near the posterior end of the body. 



Owing to the form of its constituent cells, the notochord 

 presents in transverse sections a dark central area surrounded 

 by a lighter peripheral one, but its true structure cannot be 

 unravelled without the assistance of longitudinal sections. In 

 these (PL xi. fig. 10) the nuclei form an irregular double row in 

 the centre of the cord. Their outlines are very clear, but those 

 of the individual cells cannot for certain be made out. It is, 

 however, easy to see that the cells have a flattened and wedge- 

 shaped form, with the narrow ends overlapping and inter- 

 locking at the centre of the notochord. 



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 sec- 

 tions than in transverse ones. The vacuolation of the cells pro- 

 ceeds 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 mernbrane-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. 



1 This membrane is better looked upon, as is done by Gegeubaur and Gotte, 

 as intercellular matter. 



^' 10 



