108 EMBRYOLOGY. 



one, the germinative dot (d). These different parts are 



not equally important in a physio- 

 logical point of view. The most 

 conspicuous of them, namely, the 

 shell and the white, are not essential 

 parts, and therefore are often want- 

 ing ; while the yolk, the germinative 

 Fig. 101. vesicle, and the germinative dot are 



found in the eggs of all animals, and out of these, and 



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 essential 

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

 times opaque, as in the egg of birds, sometimes transparent 

 and colorless, as in the eggs of some fishes and mollusks. 

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

 posed of an accumulation of granules. The yolk is sur- 

 rounded by a very thin skin, the vitelline membrane (Fig. 

 98, v). In some insects, when the albumen is wanting, this 

 membrane forms the exterior covering of the egg ; in such 

 cases it 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 fresh egg, near the middle of 

 the yolk, and easily recognized by the greater transparency 

 of its contents when the yolk is opaque, as in the hen's egg, 

 or by its outline, when the yolk itself is transparent, as in the 

 fish. It contains one or more little spots, somewhat opaque, 

 appearing as small dots, the germinal dots (rf). 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 white on 

 coagulation. Voluminous as it is in bird's eggs, it neverthe- 



