jjEVELOPMENT OF THE YOUNG WITHIN THE EGG. 



233 



ment of the embryo, we shall briefly describe the principal 

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

 rope, which belongs to the salmon family. The following 

 magnified 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. 



Fie. 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 

 germinative 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. 



4b'5. 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 



