OF THE EGG. 



273 



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. 



4o(i. KOKMATION 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 eirgs remain in the 

 they are very 

 minute in size. In this 

 condition they are 

 called ovarian or pri- 

 mitive eggs. They are 

 identical in all animals, 

 being, in fact, merely 

 little cells containing 

 yolk-substance (6), in- 

 cluding other similar Fig. 284. Primary ova of the bird, mag- 

 nified ; scarcely to be seen by the naked 

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

 composed of thick fibres ; c, chorion, or 

 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. 



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 nnmber, 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, or 

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

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

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

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

 number of years consecutively, generally in the spring, iu 



T 



cells, namely, the ger- 

 minative vesicle (d) 

 and the germinative 

 dot (e). The yolk it- 

 self with its membrane 

 is formed while the 



