CLEAVAGE 9 



large, segmentation is incomplete. This kind of cleavage 

 is called merohlastic cleavage. The cytoplasm may be at 

 one pole and the greater part of the egg composed of 

 deutoplasm. In eggs ha\dng this distribution of yolk 

 material, the process of segmentation is confined to the 

 region of the cytoplasm and as a result there is formed a 

 cellular disc. This kind of cleavage is called discoidal 

 cleavage. Examples of meroblastic discoidal cleavage are 

 found in the fishes, reptiles, and birds. In some eggs 

 the nucleus lies at the center and when it di\-ides the 

 nuclei thus formed migrate to the peripher\' and thus 

 the yolk becomes surrounded by a layer of cells. This 

 process is called peripheral cleavage. Examples of mero- 

 blastic peripheral cleavage are found in a number of the 

 Arthropoda. 



a. Holoblastic Cleavage 

 (i) Equal Cleavage 



On your slide of the starfish eggs look for a single un- 

 fertilized egg. The unfertilized egg may be recognized 

 by its large and comparatively clear nucleus. These 

 eggs are large enough to be seen with the naked eye. 

 Examine one first with the low power and then ^^th the 

 high power. How does the germinal vesicle compare 

 in size with the w^hole egg? WTiat difference in structure 

 do you observe between the germinal vesicle and the 

 remainder of the egg? Locate the germinal spot. 



Draw an egg about one and one-half incites in diameter 

 and label all the parts. 



Find an egg which has di\'ided. Can you find a 



