OF THE EGG. 



o- 



4. I 



example, the Podurella, are furnished with filaments which 

 give them a hairy aspect (fig. 283) ; others are cylindrical, or 

 prismatic, and frequently the surface is sculptured. 



436. FOEMATIOX 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 



/ * / v 



minute in size. In this 

 condition they are 



V 



called ovarian or pri- 

 mitive eggs. They are 

 identical in all animals, 

 being, in fact, merely 

 little cells containing 

 yolk-substance (&), in- 



Fig. 284. Primary ova of the bird, mag- 

 nified ; scarcely to be seen by the naked 

 eye ; a, stronia, or substance of the ovary, 

 composed of thick fibres ; c, chorion, cr 

 theca of the ovum, so thick as to be seen in 

 the guise of a ring ; b, yolk ; d, germinal 

 vesicle ; e, germinal spot. The structure of 

 the smaller ovum is the same. 



eluding other similar 

 cells, namely, the ger- 

 minative vesicle (d} 

 and the e;erminative 

 dot (e). The yolk it- 

 self with its membrane 

 formed while the 



s 



egg remains in the ovary ; it is afterwards enclosed in another 



envelope, the shell membrane, which may remain soft, or be 

 further surrounded by calcareous deposits, the shell proper 

 (fig. 287). 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 ; while 

 that of birds contains a much smaller number, perhaps one 

 or two hundred only. 



437. Ovulation. Having attained a certain degree of 

 maturity, which varies in different classes, the eggs leave the 

 ovary. This is called ovulation. It must not be confounded 

 with the laying of the eggs, which is the subsequent expulsion 

 of them from the abdominal cavity, either immediately, oar 

 through a special canal, the oviduct. Ovulation takes place at 

 certain seasons of the year, and never before the animal has 

 reached a particular age, which is commonly that of its full 

 growth. In a majority of species, ovulation is repeated for a 

 nn K ~- of vears consecutively, generally in the spring, in 



T 



