40 GENETICS 



nuclei Is like the set in the other, as to size and form of the 

 chromosomes, except in the case of the X and Y chromo- 

 somes above mentioned (see figures 4 and 5). 



8. The two pronuclei are at first separate; they ap- 

 proach one another, come in contact, their membranes dis- 

 solve, and the two groups of chromosomes come closer 

 together, forming a single group of 2« chromosomes (or 

 2«-i). This is the diploid group. 



9. Later this group becomes surrounded by a single 

 membrane, constituting thus the single nucleus of the ferti- 

 lized egg, containing the diploid number of chromosomes. 



10. Thus In the single nucleus of the fertilized egg, half 

 of the chromosomes are from the mother, half (or one 

 less) from the father (figures 4 and 5). 



11. Sooner or later the corresponding chromosomes of 

 the paternal and maternal sets mate or conjugate, forming 

 a pair. In some animals, as in the flies, this pairing of the 

 chromosomes takes place soon after the union of the two 

 pronuclei (in the 2-cell stage). 



12. Thus the nucleus of the fertilized egg contains n 

 pairs of chromosomes, one member of each pair paternal 

 in origin, the other maternal. 



13. In some organisms (the flies and others), the two 

 chromosomes of a pair (maternal and paternal) remain 

 paired throughout the life of the individual, and in all his 

 cells. 



14. In other species the two members do not pair till 

 later (when germ cells are formed). But finally they mate, 

 in practically all organisms. 



We shall later examine what happens in mating of the 

 chromosomes. 



15. Even in species in which the maternal and paternal 

 members are not at all times side by side, it Is often possible 

 to determine under the microscope which two chromosomes 

 form a pair, since the pairs are of different sizes and forms. 



