478 - Heredity and Evolution 



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ZYGOTE 



EMBRYONIC 

 DEVELOPMENT 



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PRIMORDIAL 

 GERM CELLS 



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TESTIS 



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SPERMATIDS • • 

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OVARY 



AUXOCYTES 

 GROWTH PERIOD 



MATURATION 



DIVISIONS 



(MEIOSIS) 



1 OOCYTE 



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POLAR BODIES 



EGG 



ZYGOTE 



Fig. 26-3. History of the germ cells in multicellular animals; germ cells black, somatic 

 cells white. The number of cell divisions is actually much greater than shown here 

 (except in the maturation period). (Adapted from Wilson.) 



pronounced; and due to an accumulation of plasm among the four resulting daughter 



yolk, the primary oocyte becomes very much cells (Fig. 26-3). One cell, which is the ripe 



larger than the primary spermatocyte. But egg, receives virtually all the accumulation 



more important still is the fact that the two of yolk and cytoplasm, whereas the polar 



meiotic divisions that follow are highly bodies receive only an insignificant fraction 



unequal as to the apportionment of cyto- (Fig- 26-5). Thus the ripe egg, if fertilized, is 



