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TEXTBOOK OF ZOOLOGY 



Disymmetrical cleavage is found in only one group of animals, the 

 Ctenophora, and is so specialized that it may be omitted from the 

 discussion here. 



In eggs having bilateral cleavager a degree of organization of the 

 egg substances is present only in the single-celled condition, a fact 

 which is discoverable in various ways in different kinds of eggs be- 

 longing here. The restriction of yolk to certain portions of the egg,- 

 pigmentation in the outer layers, or granular cytoplasm as distin- 

 guished from clear areas may be the indications of bilaterality in 



Fig. 415. — Comparison of radial and spiral cleavage. A, radial type; B, spiral 

 type in third cleavage; C and D, radial and spiral types respectively in fourth 

 cleavage. (Reprinted by perinission from Outline of Comparative Emhryology by 

 Richards, published by John Wiley and Sons, Inc., after Korschelt and Heider.) 



the egg even before the first cleavage plan appears. There can only 

 be one plane which will divide the egg into tAvo equal halves in exam- 

 ples of this kind and, as a result, the two blastomeres contain 

 approximately equal amounts of all the egg su])stances. Again the 

 second plane is at right angles to the first and the egg is divided into 

 quarters. In some animals the order of sequence of these two planes 

 is interchanged, but in the four-cell stage of all bilateral eggs two of 

 the cells must belong to the right and two to the left half of the 



