DEVELOPMENT OF THE YOUNG WITHIN THE EGG. 145 



transparent, they do not require to be cut open, and, by 

 sufficient caution, the whole series of embryonic changes 

 may be observed upon the same individual, and thus the suc- 

 cession in which the organs appear be ascertained with pre- 

 cision ; whereas, if we employ the eggs of birds, which are 

 opaque, we are obliged to sacrifice an egg for each obser- 

 vation. 



306. To illustrate these general views as to the develop- 

 ment of the embryo, we will 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. 



Fig. 111. Fig. 112. Fig. 113. 



307. The egg, when laid, (Fig. ill,) is spherical, about the 

 size of a small pea, and nearly transparent. It has no albu- 

 men, and the shell membrane is so closely attached to the 

 membrane of the yolk, that they cannot be distinguished. 

 Oil-like globules are scattered through the mass of the yolk, 

 or grouped into a sort of disk, under which lies the germina- 

 tive vesicle. The first change in such an egg occurs a few 

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

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

 sorption of a quantity of water, (Fig. 112,) by which the 

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

 and the yolk, (?/,) there is now a considerable transparent 

 space, which corresponds, in some respects, to the albumen 

 found in the eggs of birds. 



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



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