1 88 DEVELOPMENT OF FISHES 



an upper and a lower zone : the latter rich orange in colour, 

 caused by the settling of the heavier yolk material ; the 

 former lighter in colour, containing the nucleus of the ^^gy 

 and originating the growth processes. 



The less the amount of yolk in the lower, or vegetative, 

 region, the smaller is naturally the ^gg, and the more 

 obscure becomes the limit of the upper zone, or germ, or 

 animal pole, as it is indifferently called. In the yolk- 

 filled ^gg of the shark, on the other hand, the upper zone 

 becomes reduced to a mere ''germ disc" on the surface of 

 the ^gg (Fig. 216, GD). If but little yolk is present, the 

 early growth processes, i.e. the splitting of the germ cell, 

 or ^gg, into many cells, or blastomeres, to give rise to the 

 embryo, affect the entire ^gg. If, however, much yolk is 

 present, the cells at first multiply only at the animal pole, 

 and the yolk-filled region, remaining unsegmented, fur- 

 nishes the nutriment for the cell growth above. 



In the present outline of the development of fishes, 

 the following types are reviewed : — 



- I. Petromyzon ; II. Shark; III. Lung-fish; IV. Ganoid; 

 V. Teleost. 



I. The Development of Petro7nyzo7i 



The Qgg of Petromyzon is of small size (Fig. 188), and 

 is poorly provided with yolk material ; in surface view one 

 can only distinguish the germinal from the yolk region by 

 its slightly lighter colour. In the side view of the &gg of 

 Fig. 200, the beginning of the first cleavage plane is seen ; 

 a vertical plane, passing through the ^ggy completes the 

 stage of the two blastomeres of Fig. 201. The nuclei were 

 at first close to the upper, or animal, pole, but they shortly 

 take their position somewhat above the plane of the egg's 

 equator. A second cleavage plane is again vertical, ap- 



