MODIFICATIONS OF OVUM 



565 



as germinal spots, the entire nucleus of the ovum being 

 sometimes called the germinal vesicle. In other words 

 the egg, as we have already seen, is a cell. An investing 

 vitelline membrane (v. m) is usually present, though often 

 only formed after fertilisation. 



The young or immature ova of all animals present 

 this structure, but in many cases certain modifications 

 are undergone before the egg is fully formed. For 



shm 



alb' 



FIG. 149. Semi-diagrammatic view of the egg of the Fowl at the beginning of 



incubation. (Nat. size.) 



a. air-space ; alb. dense layer of albumen ; alb' more fluid albumen ; bl. blastoderm ; 

 ch. chalaza, a twisted cord of the dense layer of albumen at either end of the 

 egg, formed as the latter rotates down the oviduct ; sh. shell ; sh. m. shell- 

 membrane ; sh. m. 1, sh. m. 2, its two layers separated and enclosing air-cavity. 

 (From Parker and Has well's Zoology, after Marshall, slightly altered.) 



instance, the protoplasm may throw out pseudopods, 

 the egg becoming amoeboid (p. 307) ; or, as mentioned 

 above, an investing membrane may be secreted by the 

 egg, or by the follicular cells : this is often of consider- 

 able thickness, and may be perforated at one pole by 

 an aperture, the micropyle (p. 415 and Fig. 151, B, C). 

 The most extraordinary modification takes place in 

 some Vertebrata, such as dogfishes (p. 470) and birds. 

 In a hen's egg, for instance (Fig. 149), the yolk-granules 



