136 ' NOTOCHORD. 



no instances in which these spherules were present have been 

 met with in the late stages of development ; and they seem 

 therefore to be confined to the first stages. 



NotocJiord, 



The changes undergone by the notochord during this period 

 present considerable differences according to the genus ex- 

 amined. One iy^Q of development is characteristic of Scyllium 

 and Pristiurus ; a second type, of Torpedo. 



My observations being far more complete for Scyllium and 

 Pristiurus than for Torpedo, it is to the two former genera 

 only that the following account applies, unless the contrary 

 is expressly stated. Only the development of the parts of 

 the notochord in the trunk are here dealt with ; the cephalic 

 section of the notochord is treated of in a subsequent section. 



During stage G the notochord is composed of flattened 

 cells arranged vertically, rendering the histological characters 

 of the notochord difficult to determine in transverse sections. 

 In longitudinal sections, however, the form and arrangement 

 of the cells can be recognised with great ease. At the beginning 

 of stage G each cell is composed of a nucleus invested by 

 granular protoplasm frequently vacuolated and containing in 

 suspension numerous yolk-spherules. It is difficult to deter- 

 mine whether 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. Tlie notochord 

 thus becomes composed during stages H and I (PL x. 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 



