104 



EMBRYOLOGY. 



Fig. 96. 



Fig. 97. 



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

 of an elongated spheroid ; 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 in other 



animals. In most instances, on the 

 con trary, they are spherical, espe- 

 cially among the lower animals. 

 Fig. 95. 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 polyps, are thickly covered 

 with prickles (Fig. 96). Those 

 of certain insects, for example the 

 Podurella, 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, namely, the ovaries, which are glands, 

 ordinarily situated in the abdominal cavity. So long as they 

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

 condition they are called ovarian, or primitive eggs. 

 They are nearly the same in all animals, and 

 are in fact merely little cells containing yolk- 

 substance (?/), including other similar cells, 

 namely, the germinative vesicle (g), and the 

 germinative dot (rf). The yolk-substance it- 

 self is deposited in the ovary, and afterwards 

 enclosed in ceils. The number of these eggs is large in 

 proportion as the animal stands lower in the class to which it 

 belongs. The ovary of a herring contains more than 

 25,000 eggs ; whilst that of birds contains a much smaller 

 number, perhaps one or two hundred. 



Fig. 98. 



