188 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 egg, 
and originating the growth processes. 
The less the amount of yolk in the lower, or vegetative, 
region, the smaller is naturally the egg, 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 egg of the shark, on the other hand, the upper zone 
becomes reduced to a mere “germ disc”’ on the surface of 
the egg (Fig. 216, GD). If but little yolk is present, the 
early growth processes, z.e. the splitting of the germ cell, 
or egg, into many cells, or blastomeres, to give rise to the | 
embryo, affect the entire egg. 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 Petromyzon 
The egg 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 egg of 
Fig. 200, the beginning of the first cleavage plane is seen ; 
a vertical plane, passing through the egg, 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- 
