202 DEVELOPMENT OF GANOID 



the tail mass has hardly emerged from the surface. Pro- 

 nephros has here appeared at PN (cf. with Fig. 247, Fig. 

 210). It is not until the stage of the late embryo of Fig. 

 248 that the hinder trunk region and tail come to be 

 prominent. The embryo's axis elongates and becomes 

 straighter ; the yolk mass is now much reduced, acquiring 

 a more and more oblong form, lying in front of the tail, T, 

 in the region of the posterior gut (cf. Figs. 211 and 212). 

 The head, and even the region of the pronephros, PN, 

 are clearly separate from the yolk sac ; the mouth, My is 

 coming to be formed. 



IV. The Development of Ganoids 



The development of Ganoids is next to be outlined. 

 The eggs of the sturgeon and gar-pike are poorly provided 

 with yolk. They have still, however, a greater amount 

 than those of the lamprey or lung-fish, and in many 

 regards of development suggest nearnesses to the Elasmo- 

 branchs. 



The egg of the sturgeon shown in Fig. 249 shows 

 clearly two distinct zones; the upper, blotched with pig- 

 ment at the animal pole, is pale in colour; the lower, rich 

 in yolk, is orange-coloured, well speckled with pigment. 

 The early cleavages appear at first only in the upper pale- 

 coloured area which corresponds apparently with the germ 

 disc of the shark's ^gg. In Fig. 250 there have been 

 two cleavages, vertical and at right angles to each other ; 

 these have sharply traversed the germ area, the earlier 

 one being now produced slightly into the yolk region of 

 the ^gg — only, however, as a slight surface furrow. The 

 third cleavage (Fig. 251) presents a stage closely corre- 

 sponding with that of Ceratodus of Fig. 235, its plane tend- 

 ing to pass parallel to the first cleavage : the germ disc 



