138 EMBRYOLOGY. 



The yolk is surrounded by a very thin skin, the vitelline 

 membrane, (Fig. 98, v.) In some insects, when the albumen 

 is wanting, this membrane, surrounded by a layer of pecu- 

 liar cells, forms the exterior covering of the egg, which, in 

 such cases, is generally of a firm consistence, and sometimes 

 even horny. 



288. The germinative vesicle (Fig. 98, g) is a cell of ex- 

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

 of the yolk, and easily recognized by the greater transpar- 

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

 as in the hen's egg, or by its outline, when the yolk itself is 

 transparent, as in eggs of fishes and mollusks. It contains 

 one or more little spots, somewhat opaque, appearing as 

 small dots, the germinal dots, (d.) On closer examination, 

 these dots are themselves found to contain smaller nucleoli. 



289. The albumen, or white of the egg, (Fig. 101, a,) is 

 a viscous substance, generally colorless, but becoming 

 opaque white on coagulation. Voluminous as it is in birds' 

 eggs, it nevertheless plays but a secondary part in the histo- 

 ry of their development. It is not formed in the ovary, like 

 the yolk, but is secreted by the oviduct, and deposited around 

 the yolk, during the passage of the egg through that canal. 

 On this account, the eggs of those animals in which the ovi- 

 duct is wanting, are generally without the albumen. In 

 birds, the albumen consists of several layers, one of which, 

 the clialazcB, (c,) is twisted. Like the yolk, the albumen is 

 surrounded by a membrane, the shell membrane, (w,) which 

 is either single or double, and in birds, as also in some 

 reptiles and mollusks, is again protected by a calcareous 

 covering, forming a true shell, (s.) In most cases, how- 

 ever, this envelop continues membranous, particularly in the 

 eggs of the mollusks, most crustaceans and fishes, salaman- 

 ders, frogs, &c. Sometimes it is horny, as in the sharks 

 and skates. 



