THE NOTOCHORD. 355 



there is only one vacuole for each cell, or whether in some cases 

 there may not be more than one. 



Round the exterior of the notochord there is present a 

 distinct though delicate cuticular sheath. 



The vacuoles are at first small, but during stage G rapidly 

 increase in size, while at the same time the yolk-spherules 

 completely vanish from the notochord. 



As a result of the rapid growth of the vacuoles, the nuclei, 

 surrounded in each case by a small amount of protoplasm, 

 become pushed to the centre of the notochord, the remainder of 

 the protoplasm being carried to the edge. The notochord thus 

 becomes composed during stages H and I (PI. n, fig. 4 6) of a 

 central area mainly formed of nuclei with a small quantity of 

 protoplasm around them, and of a thin peripheral layer of 

 protoplasm without nuclei, the widish space between the two 

 being filled with clear fluid. The exterior of the cells is 

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

 brane 1 ; 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 haema- 

 toxylin. Their long diameter in Scyllium is about O'O2 Mm. 



The diameter of the whole notochord in Pristiurus during 

 stage I is about O'i Mm. in the region of the back, and about 

 O'oS 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 12, 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 interlocking 

 at the centre of the notochord. 



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

 intercellular matter, 



