PROTOVUM AND METOVUM. 



223 



the so-called Graafian follicle ; immediately under this the 

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



At a very early period the small protovum of the Bird 

 begins to imbibe a mass of food-substance through the 

 yelk-membrane, and to elaborate this matter into the so- 

 called " yellow yelk." The protovum is thus transformed 

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

 than the protovum, but which, nevertheless, is only a single, 

 enormously enlarged cell. 72 The accumulation of the large 

 yellow-yelk mass within the ball of protoplasm forces the 

 kernel (vesicula germinativa), which is contained in the 

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

 the kernel (vesicula germinativa) is surrounded by a small 

 quantity of protoplasm ; and these two together form the 

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

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

 point of the upper surface, in the form of a small, white, 

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



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

 ovary of a Hen (in section). The yellow 

 nutritive yelk is composed of concentric 

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

 membrane (a). The cell-kernel (vesicula germi- 

 nativa), together with the protoplasm of the 

 egg-cell, forms the formative yelk (b), or the 

 tread. The white yelk (here represented as 

 black) passes from the tread to the yelk- 

 cavity (cT). The two kinds of yelk are, 

 however, not shai-ply distinguished. 



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

 no particles of yellow yelk, and is softer than the yellow 

 nutritive yelk, passes from the tread directly to the 



