The Development of the Chick 99 



amount of yolk be evenly distributed through- 

 out the egg, the cleavage will be equal, as in 

 the eggs of Amphioxus and Mammals. If the 

 yolk be rather more abundant, and be concen- 

 trated nearer one pole (vegetative) of the egg, 

 the cleavage will be unequal (usually only after 

 the third division), as in the eggs of Cyclo- 

 stomes and Amphibia. 



Partial cleavage takes place in eggs which 

 are often very large, and which contain a large 

 amount of yolk. This food yolk is so un- 

 equally distributed that the egg contents may 

 be divided into a formative yolk, in which 

 alone the process of cleavage takes place, and 

 a nutritive yolk, which does not divide and 

 which serves as food for the nourishment of the 

 growing embryo. If the food yolk be accu- 

 mulated at one pole of the egg segmentation 

 will be confined to a disc-shaped area of forma- 

 tive yolk situated at the opposite or animal 

 pole ; this is the discoidal form of cleavage 

 found in Fishes, Reptiles, and Birds. 



When the food yolk Is collected at the centre 

 of the egg, with the formative yolk Investing 

 it, the cleavage will be superficial. The nucleus 

 is, in this case, usually in the centre of the egg, 

 and the daughter nuclei which are formed by 



