276 OF THE EGG. 



climates dispense with this task ; the ostrich, for example, 

 often contents herself with depositing her eggs in the sand 

 of the desert, leaving them to be hatched by the sun. In 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 stickle-backs, sun-fishes, and cat-fishes ; but 

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

 entering into the details of embryonic transformations, a few 

 words are necessary respecting the composition of the egg. 



443. Composition of the Egg. The egg is composed of 

 several substances, varying in structure, as well as in appear- 

 ance. Thus, in a new-laid hen's egg (fig. 287), we have first 

 a calcareous shell lined by a double membrane, the shell mem- 

 brane (c) ; then an albuminous substance, the white ; in which 

 several layers may be distinguished (ef) ; within this, we find 

 the yolk enclosed in its membrane (h) ; and before it was laid, 

 there was in the midst of the latter a minute vesicle, the ger- 

 minative vesicle (fig. 284, c?), containing a still smaller one, 

 the germinative dot (e). These different parts are not equally 

 important in a physiological point of view. The most con- 

 spicuous of them, namely, the shell and the white, are not 

 essential parts, and therefore are often wanting; while the 

 yolk, the germinative vesicle, and the germinative 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 in figs. 284 287. 



444. The vitellus,or yolk (fig. 287, A), is the most essen- 

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

 sometimes opaque, as in the egg of birds, sometimes transpa- 

 rent and colourless, as in the eggs of some fishes and mollusca. 

 On examination under the microscope, it appears to be com- 

 posed of an accumulation of granules and oil drops. The yolk 

 is surrounded by a very thin skin, the vitelline membrane (fig. 

 284, c). In some insects, when the albumen is wanting, this 

 membrane, surrounded by a layer of peculiar cells, forms the 

 exterior covering of the egg ; which in such cases is generally 

 of a firm consistence, and sometimes even horny. 



445. The germinative vesicle (fig. 284, d) is a cell of ex- 

 treme delicacy, situated, in the young egg, near the middle of 

 the yolk, and easily recognized by the greater transparency 

 of its contents when the yolk is in some degree opaque, as in 



