62 



GENETICS IN RELATION TO AGRICULTURE 



tion figure shown at F, the significant feature of which is the fact that each 

 of the daughter nuclei resulting from this division receives only two 

 chromosomes instead of the four which the original cell at A contained. 

 Since the original cell contained one pair of larger and one pair of smaller 

 chromosomes, the daughter cells which are formed each receive one larger 

 and one smaller chromosome. 



Cytological investigation is not yet in agreement as to the interpre- 

 tation of synapsis especially as to the manner in which the phenomena 

 therein concerned are connected with preceding mitotic divisions. Con- 

 sidering certain cytological investigations and the results of research in 



FIG. 29. The reduction division as represented for a species whose diploid number is 

 four. A, "Resting" nucleus of a primary germ cell. B, Formaton of paired threads of 

 chromomeres. C, Conjugation of homologous chromosomes (synapsis). D, Loosening 

 of the synaptic knot. E, Condensation of the chromosomes and disappearance of the nuclear 

 membrane. F , Homologous chromosomes about to pass to opposite poles, thus giving each 

 secondary germ cell a member of each pair and one-half the somatic number. 



heredity together, it appears that the threads which pair in stage B rep- 

 resent pairs of chromosomes with homologous chromomeres occupying 

 corresponding positions along their entire length. Likewise the contrac- 

 tion stage at C is taken to represent a conjugation of the members of pairs 

 of chromosomes which later again separate. Other cytological evidence 

 indicates that in some forms the conjugation of pairs of homologous chro- 

 mosomes is brought about in another way. However, the essential 

 fact is the same in either case. In the reduction figure the members of 

 each pair of chromosomes are distributed to the opposite poles of the 

 spindle so that the daughter nuclei received only one member of each pair. 

 The significance of synapsis lies in the conjugation of homologous 



