34 



ZOOLOGY 

 FIG. 12. 



MALE 



FEMALE 



^Jy)] SPERMATOGONIUM UNIVALENT CHROMOSOMES <2X) 



CHROMOSOMES UNITE IN PAIRS 



BIVALENT (X) PRIMARY OOCYTE' 



PRIMARY SPERM ATOCYTE 



MATURATION DIVISION 



SECONDARY UNIVALENT CHROMOSOMES (Xj SECONDARYJDOCYTE 

 SPERMATOCYTF 



II MATURATION DIVISION 



8. UNION OF NUCLEI 



lO, CLEAVAGE 



FIG. 12. A diagram to show the central place of the fertilized ovum, and the important stages 

 leading up to it together with those following it. Compare with Pig. n. The male chromosome 

 are figured black and the female light, throughout. Compare descriptive matter in text. 



Questions on the figure. In what respects is the maturation of the ovum and 

 of the sperm similar? In what, different? What is the probable meaning of the 

 pairing of the chromosomes (synopsis) in the early maturation stages. How does 

 this first division of chromosomes differ from the ordinary divisions. Why may 

 fhe polar bodies be called abortive eggs? What is accomplished by the union of 

 the sperm and ovum nuclei? In cleavage the chromosomes from both parents 

 seem to be distributed equally to all the cells of the body. What is the significance 

 to this? 



