OF THE EGG. 



133 



Fig. 95. 



eggs should, therefore, be considered as a universal charac- 

 teristic of the Animal Kingdom. 



277. Form of the Egg. The general form of the egg 

 is more or less spherical. The eggs of birds have the form 

 of an elongated spheroid, narrow at one end ; and this 

 form is so constant, that the term oval has been universally 



adopted to designate it. But this is by no means the usual 



form of the eggs of other animals. 



In most instances, on the contra- 

 ry, they are spherical, especially 



among the lower animals. Some 



have singular appendages, as those 



of the skates and sharks, (Fig. 95,) which are shaped like 



a hand-barrow, with four hooked horns at the corners. The 



eggs of the hydra, or fresh water 



polyp, are thickly covered with 



prickles, (Fig. 96.) Those of 



certain insects, the Podurella, for 



example, are furnished with fila- 

 ments which give them a hairy 



aspect, (Fig. 97 ;) others are cylindrical or prismatic ; and 



frequently the surface is sculptured. 



278. Formation of tlie Egg. The egg originates within 



peculiar organs, called ovaries, which are glandular bodies, 



usually situated in the abdominal cavity. So long as the 



eggs remain in the ovary, they are very minute in size. In 



this condition they are called ovarian, or primitive eggs. 



They are identical in all animals, being, in 



fact, merely little cells (v) containing yolk, 



(?/,) and including other smaller cells, the 



germinative vesicle, (g,) and the germinative 



dot, (d.) The yolk itself, with its membrane, 



(i>,) is formed while the egg remains in the 



ovary. It is afterwards enclosed in another 



envelope, the shell membrane, which may remain soft, (5,) 



12 



Fig. 96. 



Fig. 97. 



Fig. 98. 



