14 INVKRTEBRArE ZOOLOGY 



from the parent individual. The so-called middle piece of the 

 sperm cell contains a centrosome. Barring some very unusual 

 circumstances, this centrosome brought in by the male pronucleus 

 forms a mitotic spindle within which the chromatin of both the 

 male and female pronuclei becomes commingled. The mitotic 

 division of the fertilized egg which ensues is followed by cleavage 

 of the surrounding cytoplasm. Through a continued sequence 

 of mitosis and cleavage, large numbers of cleavage cells or 

 blastomeres are formed. 



Cleavage Patterns. — The relative size and arrangement of 

 these blastomeres is greatly influenced by the amount and distri- 

 bution of the stored material within the egg. If the yolk is evenly 

 distributed, the egg is said to be homolecithal and it undergoes 

 a complete cleavage resulting in the formation of numerous cells, 

 all of which are practically uniform in size. However, deuto- 

 plasm is heavier than the surrounding cytoplasm, and in many 

 instances tends to accumulate at the vegetative pole of the egg. 

 The term telolecithal is applied to such an egg. Yolk serves 

 as a mechanical obstruction to the paths of the cleavage planes. 

 Consequently, if the vegetative pole is heavily yolk laden, cleav- 

 age is restricted to a disc of cytoplasm at the animal pole. How- 

 ever, in some telolecithal eggs, the entire cell cleaves, but, since 

 the mitotic spindle tends to take a position in the center of the 

 cytoplasm of the cell, spindles will be formed nearer the animal 

 than the vegetative pole and as a consequence the resulting cells 

 are unequal in size. The cells at the animal pole are much 

 smaller than those at the vegetative pole. Some arthropod eggs 

 have the yolk collected in the center and are therefore said to 

 be centrolecithal. Cleavage in such an egg is restricted to the 

 layer of cytoplasm surrounding the yolk (Fig. 8) and is referred 

 to as superficial cleavage. 



The blastomeres resulting from cleavage of the fertilized egg 

 assume various arrangement patterns. In some groups of ani- 

 mals, the blastomeres fail to follow any orderliness in their 

 formation and arrangement. Instances of this sort are desig- 

 nated as indeterminate cleavage. This condition stands in sharp 

 contrast with that found in some other groups, the members of 

 which have cleavage processes so orderly that it is possible to 

 predict with exactitude the direction which successive cleavage 

 planes will take. Determinate cleavage, as this is called, renders 

 it possible to trace the history of each blastomere, to follow 



