36 



CYTOLOGY 



CHAP. 



pre-meiotic nuclei. As the chromosomes behave similarly in meiosis in 

 the female, the fusion of the male and female gametes results in the 

 reconstitution of a diploid nucleus with four chromosomes in two pairs. 

 Thus, if we designate each of the differently shaded chromosomes of 

 the diagram by a different letter, the gametes contain chromosomes 



B 



Fig. 15. 



Diagram of the principal stages of meiosis by parasyndesis. Two pairs of homologous chromosomes are 

 shown, the members of one pair being stippled, and those of the other cross-striped. A, pre-meiotic 

 prophase, showing the four separate chromosomes ; B, leptotene ; C, pachytene ; D, diplotene stages ; 

 E, diakinesis, showing the evolution of the definitive bivalents ; F, meiotic metaphase ; G, metaphase of 

 second division of the meiotic phase in the secondary spermatocyte formed from the upper daughter 

 nucleus derived from F. 



A -f B, and the zygote A-i-A + B + B. In syndesis pairing takes .place 

 in such a way that the pachytene nuclei contain two bivalents, forming 

 the series A A + BB. We also see that one member of each bivalent was 

 originally introduced by the male gamete and the other by the female. 



While the most important features of meiosis are all to be found 



