OF THE EGG. 



137 



J>ke mi nner, the eggs of most birds may be hatched by main- 

 taining tliem at the proper temperature by artificial means. 

 Some fishes are also known to build nests and to sit upon 

 their oggs, as the sticklebacks, sun-fishes, and cat-fishes ; but 

 whether they impart heat to them or not, is doubtful. 



Before entering into the details of embryonic transfor- 

 mations, a few words are necessary respecting the composi- 

 lion of the egg. 



286. Composition of the Egg. — The egg is composed 

 of several substances, varying in structure, as well as in 

 aj^pearance. Thus, in a hen's egg, (Fig. 101,) we have first 

 a calcareous shell., (5,) lined by a double membrane, the shell 

 membrane.^ {711;) then an albuminous substance, the white., 

 (a,) in which several layers may be distinguished ; within 

 this we find the yolk., (v/,) enclosed in its membrane ; and 

 before it was laid, there was in the midst of the latter a mi- 

 nute vesicle, the germinative vesicle., (Fig- 98, g^) containing 

 a still smaller one, the germinative dot., (d.) These difTerent 

 parts are not equally important in a 

 physiological point of view. The 

 most conspicuous of them, namely, 

 the shell and the white, are not es- 

 sential parts, and therefore are often 

 wanting ; while the yolk, the ger- 

 minative vesicle, and the germina- 

 tive dot are found in the eggs of 



all animals ; and out of these, and of these only, the germ ig 

 formed, in the position shown by Fig. 101, e. 



287. The vitcllus or yolk (Fig. 101, ?/) is the most essen- 

 tial part of the egg. It is a liquid of variable consistence, 

 sometimes opaque, as in the eggs of birds, sometimes trans- 

 parent and colorless, as in the eggs of some fishes and 

 mollusks. On examination under the microscope, it appears 

 to be composed of an accumulation of granules and oil-drops, 



12* 



Fig. 101. 



