OF THE EGG. 137 



like manner, the eggs of most birds may be hatched by main- 

 taining them at the proper temperature by artificial means. 

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

 their eggs, 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- 

 tion of the egg. 



286. Composition of the Egg. The egg is composed 

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

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

 a calcareous shell, (s,) lined by a double membrane, the shell 

 membrane, (m ;} then an albuminous substance, the white^ 

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

 this we find the yolk, (y,) 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 different 

 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- Fig. 101. 

 tive dot are found in the eggs of 



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

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



287. The vitellus or yolk (Fig. 101, y) 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* 



