DEVELOPMENT OF FISHES 193 
Il. Zhe’ Development of the Shark 
On the side of embryology a shark presents many points 
of striking contrast to the lamprey; yet it may in many 
regards be looked upon as archaic in its developmental 
characters. Its contrasting structures (together with those 
of lung-fish, Ganoid, and Teleost) may best be reviewed 
in the table, p. 280. 
The egg of the shark is of large size, richly provided 
with yolk material. When removed from its membranes, 
it is seen to be of a bright orange colour; its form is elon- 
gated, and the weight of its pasty substance causes it to 
assume a flattened ovoid (Fig. 216). At the upper pole of 
the egg is a small, light-coloured spot, the germ disc, GD, 
which figures prominently in the early stages of develop- 
ment. It would represent the lamprey’s entire egg, if one 
could imagine a point of the lower pole of the latter hugely 
dilated with yolk. It is in the region of this germ disc 
alone that every process of development as far as gastrula- 
tion occurs. 
The segmentation of the germ disc is shown in Figs. 
217-220. In the first of these (Fig. 217) the germ is seen 
to be sharply marked off from the surrounding yolk by a 
circular band ; two cleavages have traversed it in the form 
of narrow grooves separating the blastomeres. In Fig. 
218 the fifth cleavage has been completed; the furrows 
dividing irregularly the surface of the germ disc fade away 
at its periphery. Fig. 219 represents a vertical section of 
the germ disc at this stage ; the upper, finely dotted layer, 
thinning away at either side, is the germ disc ; the coarsely 
granular material below is the yolk; the depth of the 
cleavage furrows is seen, and it will be noted that up to 
this stage of development there have been no horizontal 
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