DEVELOPMENT OF THE TOTING WITHIN THE EGG. 



283 



ment of the embryo, we shall briefly describe the principal 

 phases, a? they have been observed in the white-fish of Eu- 

 rope, which belongs to the salmon family. The following 

 unified sections will illustrate this development, and show 

 the^period at which the different organs successively appear. 



464. The egg when laid (fig, 297) is spherical, about the 

 size of a small pea, and nearly transparent. 



Fig. 297. 



Fig. 298. 



Fig. 299. 



It has no albumen, and the shell-membrane is so closely at- 

 tached to the membrane of the yolk, that they cannot be dis- 

 tinguished. Oil-like globules are scattered through the mass of 

 the yolk, or grouped into a sort of disc, under which lies the 

 gerininative vesicle. The first change in such an egg occurs a 

 few hours after it has been laid, when the shell-membrane 

 separates from the yolk-membrane, in consequence of the ab- 

 sorption of a quantity of water (fig. 298), by which the 

 egg increases the size. Between the shell-membrane (s, m) 

 and the yolk (y) there is now a considerable transparent 

 space, corresponding, in some respects, to the albumen found 

 in the eggs of birds. 



465. Soon afterwards we see, in the midst of the oil-like 

 globules, a swelling in the shape of a transparent vesicle 

 (fig. 299, g), composed of very delicate cells. This is the 

 first indication of the germ. The swelling rapidly enlarges 

 until it envelops a large part of the yolk, when a depression is 

 formed in it (fig. 300). This depression becomes by degrees 



Fig. 300. 



Fig. 301. 



Fig. 302. 



a deep furrow, and soon after a second furrow appears at 

 right angles with the former, so that the germ now presents 



