BIOLOGY OF EGGS AND IMPLANTATION 



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tilized egg. In the mouse the mitochondria 

 and Golgi material of the sperm become dis- 

 persed throughout the egg cytoplasm and 

 the axial filament of the flagellum disap- 

 pears before the first cleavage. But in the rat 

 the flagellum is of such length and rigidity 

 that it assumes an eccentric position within 

 the periphery of the cell. Probably this ex- 

 plains why the male pronucleus ordinarily 

 begins its development in the outer zones 

 of the egg. Between the 15th and 19tli hour 



after penetration, the external sheaths of 

 the middle- and main-pieces begin to lose 

 their smooth contours and they gradu- 

 ally disappear (Fig. 14.15). When this has 

 been accomplished, the spiral mitochondrial 

 sheath of the middle piece and the axial 

 filament of the main piece can be clearly 

 visualized. Immediately before the first 

 cleavage, the continuous helical mitochon- 

 drial thread begins to swell. During the 

 2-cell stage, the mitochondrial thread is 





Fig. 14.15. Chromosomes from the metaphase of the first segmentation division removed 

 from a living, fertilized rat egg, 1. The sperm flagellum from the same egg lies just below the 

 chromosomes. Note that the spiral mitochondrial sheath (SMP) is still present. At 2, two- 

 cell rat egg with the remains of the sperm flagellum at arrow. At 3, unfertilized rat egg in 

 which the fragments appear similar to the blastomeres of a normally fertilized egg but there 

 is no sperm flagellum present (Blandau and Odor, 1952). 



