DEVELOPMENT OF FISHES 



II. The Development of the Shark 



193 



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 Qgg, 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 



