FROM EGG TO INSECT — SNODGRASS 



379 



more cells like the original ones. But finally there comes a time 

 when a new kind of division intervenes which, instead of preserving 

 the original number of chromosomes, reduces it. The chromosome 

 threads may appear in the ordinary manner and in the usual num- 

 ber, but they develop an attraction for one another which leads them 

 to come together and unite in pairs (fig. 4 A, B, C). This mating 

 of the chromosomes is called synapsis (from two Greek words mean- 

 ing a union), and a most interesting thing about it is that it con- 

 sists, not of a haphazard union of any chromosome with any other, 

 but of a definite pairing of corresponding male and fenuile chro- 

 mosomes descended from the parrent chromosomes that came together 





Fig. 4. — Diagram of synapsis, or the mating of the chromosomes prior to the 



maturation divisions 

 A-C, synapsis in a female germ cell nucleus with six chromosomes, including 

 two x-chromosomes {x, w), resulting iu a nucleus (C) with tliroe double chromo- 

 somes, including one double x-chromosome ; D-F, synapsis in a male germ cell 

 nucleus with four orthochromosomes and one x-chromosome, resulting in a 

 nucleus (F) with two double orthochromosomes, but with one single x-chromo- 

 some. 



at the fertilization of the egg from which this generation of germ 

 cells was derived. At least, there is very good evidence that this 

 is what takes place; it is even suggested by some writers that there 

 is an interchange of substance between the chromosomes while 

 mated, thus bringing about a redistribution of hereditary characters 

 carried by the chromosomes. 



We have seen that the chromosomes are often distinguished by 

 visible differences in size or shape. Two of the chromosomes in the 

 female egg cell before synapsis are of special interest, for reasons 

 that will appear later, and are usually referred to as the female 



