94 Vertebrate Embryology 



Maturation of the Egg 

 Since the processes of maturation in the 

 chick take place long before the egg is laid, it 

 is very difficult to work them out, and they are 

 imperfectly known : but it is probable that the 

 changes that take place are more or less simi- 

 lar to those that have been briefly described in 

 speaking of the maturation of the frog's egg. 

 The nucleus or germinal vesicle of the grow- 

 ing ovum is large, and lies near the centre of 

 the egg ; but as the egg matures, the nucleus 

 moves towards the surface and eventually lies 

 just beneath the vitelline membrane, in a len- 

 ticular area, the germinal disc, which is nearly 

 free from food yolk. 



When fully ripe, the egg bursts from the 

 ovary into the body-cavity, and enters the 

 funnel-like end of the oviduct. As it passes 

 through the upper or thin-walled part of the 

 oviduct, it has secreted around it, by the walls 

 of the oviduct, the white or albuminous en- 

 velope, in spiral bands ; the spiral arrangement 

 being especially evident in the more dense 

 albumen of the chalazse. This spiral arrange- 

 ment is caused by the rotation of the ovum by 

 the spirally arranged folds in the upper part of 

 the oviduct. It probably takes about three 



