34 



ZOOLOGY 

 Pig. 12. 



FEMALE 



^ 'Ty]\ SPERMATOGONIUM UNIVALENT CHROMOSOMES (2X) OOGONIUM ( K" ^ 



CHROMOSOMES UNJTE IN PAIRS 



BIVALENT IX) PRIMARY OOCYTE 



PRIMARY SPERMATOCYTE 



I MATURATION DIVISION. 



(fj) j SECONDARY UNIVALENT CHROMOSOMES (X) SECONDARY OOCYTE 



II MATURATION DIVISION 



SPERMATIDS ■* SPERMATOZOA 



8. UNION OF NUCLEI 



la CLEAVAGE 



Fig. 13. A diagram to show the central place of the /ffrft'Itsed oi'Uffl, and the important stages 

 leading up to it together with those following it. Compare with Fig. ii. 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 (synapsis) in the early maturation stages. How does 

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

 the 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 

 of this? How much of Fig. li does Pig. I2 cover? 



