PROTOVUM AND METOVUM. 



223 



le so-called Graafian follicle ; immediately under this the 

 ructureless yelk-membrane is secreted by the egg-yelk. 



At a very early period the small protovum of the Bird 

 gins to imbibe a mass of food-substance through the 

 5lk-membrane, and to elaborate this matter into the so- 

 iled " yellow yelk." The protovuTn is thus transformed 

 .to the metovum (after-egg), which is many times larger 

 lan the protovum, but which, .nevertheless, is only a single, 

 lormously enlarged cell.'^ The accumulation of the large 

 }llow-yelk mass within the ball of protoplasm forces the 

 mel {yesicula gerniinativa), which is contained in the 

 tter, quite to the upper surface of the yelk-mass. Here 

 16 kernel {vesicula germinativa) is surrounded by a small 

 lantity of protoplasm ; and these two together form the 

 ntil-shaped " formative yelk " (Fig. 44, b). This appears 



the outside of the yellow yelk-mass, at a particular 

 )int of the upper surface, in the form of a small, white, 

 rcular point ; the so-called " tread," or cicatricula. A 



Fig. 44. — A mature egg-cell from the 

 ^ary of a Hen (in section). The yellow 

 atritive yelk is composed of concentric 

 lyers (c), and is surrounded by a thin yelk- 

 lembrane (a). The cell-kernel {vesicula gernii- 

 }iiira), together with the protoplasm of the a <;• 

 j-cell, forms the forhnative yelk (b), or the 

 Bad. The white yelk (here represented as 

 lack) passes from the tread to the yelk- 

 jivity (d'). The two kinds of yelk are, 

 Wever, not sharply distinguished. 



aread-like cord of white nutritive yelk (d), which contains 

 !o particles of yellow yelk, and is softer than the yellow 

 utritive yelk, passes from the tread directly to the 



