The Development of the Chick 97 



in this case, also, the blastomeres are of unequal 

 size, the segmentation is said to be unequal. 

 A more common form of cleavage, seen, for 

 example, in the starfish, is where the entire 

 egg divides into equal blastomeres ; this is 

 known as complete and equal segmentation. 

 The holoblastic form of cleavage is common 

 in the case of small eggs where the food yolk 

 is in small quantity. 



In other eggs, such as those of Birds, Rep- 

 tiles, and many Arthropods, we have what is 

 known as meroblastic or partial cleavage ; in this 

 form of segmentation the cleavage planes do not 

 extend entirely through the egg, so that only 

 a part of the egg is divided into the segments 

 or blastomeres. In the Arthropods there is a 

 superficial layer of protoplasm entirely round 

 the egg that is free from yolk, and it is in 

 this layer that segmentation takes place ; such 

 a form of partial segmentation is known as 

 superficial. 



In the chick we have an example of what is 

 known as discoidal segmentation. Instead of 

 having, as in the Arthropods, a layer of yolk- 

 free protoplasm entirely round the egg, there 

 is, in the chick, a small disc-shaped area, the 

 above-mentioned germinal disc, lying on one 



