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 the 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 (u) containing yolk, 

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

 germinative vesicle, (g,) and the germinative 

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

 (y,) is formed while the egg remains in the 

 ovary. It is afterwards enclosed in another 

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



12 



Fig. 96. 



Fig. 97. 



Fig. 98. 



