12 SEGMENTATION. 



In the next youngest of the eggs* examined the germinal 

 disc was already divided into twenty-one segments. AVhen 

 viewed from the surface (PI. I. 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 I. 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 

 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, PI. I. fig. 6, bears out the state- 

 ments 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 observed an 



example of two furrows crossing one another at this stage. 



^ The germinal disc figured was from the egg of a Scy Ilium stellare and not 

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

 do not differ materially from the Scyllium sections. 



