REPRODUCTION 



405 



way before the egg is laid. It is always on that side 

 which naturally turns uppermost, for the yolk can turn 

 upon its axis ; it is, therefore, always nearest to the 

 external air and to the hen's body two conditions 

 necessary for its development. There is another reason 

 for this polarity of the egg : the lighter and more deli- 

 cate part of the yolk is collected in its upper region, 

 while the heavy, oily portion remains beneath. 



In most eggs the shell and albumen are wanting. 

 When the albumen is present, it is commonly covered 

 by a membrane only. In sharks the envelope is horny ; 

 and in crocodiles is calcareous, as in 

 birds. 



The egg of the sponge has no true 

 vitelline membrane, and is not unlike 

 an ordinary amoeboid cell. An egg 

 is, in fact, little more than a very large 

 cell, of which the germinal vesicle is 

 the nucleus. 



The size of an egg depends mainly FIG. 359 . - E gg of Sponge 

 upon the quantity of yolk it contains ; (magnifi< i: " nucleus ' 

 and to this is proportioned the grade of development 

 which the embryo attains when it leaves the egg. 162 In 

 the eggs of the starfishes, worms, insects, mollusks (ex- 

 cept the cuttlefishes), many amphibians, and mammals, 

 the yolk is very minute and formative, i.e., it is con- 

 verted into the parts of the future embryo. In the 

 eggs of lobsters, crabs, spiders, cephalopods, fishes, 

 reptiles, and birds, the yolk is large and colored, and 

 consists of two parts the formative, or germ yolk, 

 immediately surrounding the germinal vesicle ; and the 

 nutritive, or food yolk, constituting the greater part of 

 the mass, by which the young animal in its egg life is 

 nourished. In the latter case, the young come forth 

 more mature than when the food yolk is wanting. 



