224 DEVELOPMENT OF ELASMOBRANCH FISHES. 



In the next youngest of the eggs 1 examined the germinal 

 disc was already divided into twenty-one segments. When 

 viewed from the surface (PI. 6, fig. 3), the segments appeared 

 divided into two distinct groups an inner group of eleven 

 smaller segments, and an outer group of segments surrounding 

 the former. The segments of both the inner and the outer 

 group were very irregular in shape and varied considerably in 

 size. The amount of irregularity is far from constant and many 

 germinal discs are more regular than the one figured. 



In this case the situation of the germinal disc and its relations 

 to the yolk were precisely the same as in the earlier stage. 



In sections of this germinal disc (PL 6, fig. 6), the groove 

 which separates it from the yolk is well marked on one side, but 

 hardly visible at the other extremity of the section. 



Passing from the external features of this stage to those 



o o 



which are displayed by sections, the striking point to be noticed 

 is the persisting continuity of the segments, marked out on the 

 surface, with the floor of the germinal disc. 



The furrows which are visible on the surface merely form a 

 pattern, but do not isolate a series of distinct segments. They 

 do not even extend to the limit of the finely granular matter of 

 the germinal disc. 



The section represented, PL 6, fig. 6, bears out the statements 

 about the segments as seen on the surface. There are three 

 smaller segments in the middle of the section, and two larger 

 at the two ends. These latter are continuous with the coarser 

 yolk-spheres surrounding the germinal disc and are not separated 

 from them by a segmentation furrow. 



In a slightly older embryo than the one figured I met with 

 a few completely isolated segments at the surface. These 

 segments were formed by the apparent bifurcation of furrows 

 as they neared the surface of the germinal disc. The segments 

 thus produced are triangular in form. They probably owe 

 their origin to the meeting of two oblique furrows. The last- 

 formed of these furrows apparently ceases to be prolonged 

 after meeting the first-formed furrow. I have not in any case 



1 The germinal disc figured was from the egg of a Scyllium stellare and not 

 Pristiuvus, but I have also sections of a Pristiurus egg of the same age, which do 

 not differ materially from the Scyllium sections. 



