THE REDUCTION OF THE CHR0M0S0M1 25] 



An interpretation put forward by Bonnevie (1906, L908) is shown In 

 Fig. 99, C. Here the chromosomes conjugate parasynaptically and 

 come into very intimate union: although fchey appear to undergo a real 

 fusion their identity is maintained for a time. Owing to the fact thai 

 these bivalent chromosomes are inserted upon the Bpindle with their 

 halves in juxtaposition (side-by-side with respect to the poles) rather than 

 in superposition (one toward each pole), the members of a conjugated 

 pair separate neither at the first division nor at the second. As a result 

 each of the four cells receives the haploid number of chromosomes, all 

 of which are bivalent, and no qualitative reduction occurs. Bonnevie 

 believes that the conjugating members of each pair finally fuse completely 

 in the subsequent stages. In this case, therefore, as in the preceding one 

 numerical reduction is supposed to result from a complete fusion of the 

 chromosomes in pairs. 



Whether any confidence is to be placed in such interpretations or not 

 — and according to most cytologists none should be — they at least serve 

 to show how it is possible that numerical reduction may occur without 

 effecting any qualitative reduction, and that the essential feature of tin 

 reduction of the chromosomes is something other than the mere change 

 in their number, as pointed out at the beginning of the chapter. 



SYNAPSIS, OR CHROMOSOME CONJUGATION 



The phenomenon of chromosome conjugation, or synapsis, which we 

 have seen above is such an important feature of the reduction process, 

 must now be somewhat more closely examined. Attention will !><• 

 directed to three points: the relationship of the conjugating memfo 

 (the "synaptic mates"), the stage at which the synaptic union taki - 

 place, and the exact nature of this union. 



Relationship of the Synaptic Mates.— We may first inquire into tin 

 relationship which may exist between the two chromosomes pairing to 

 form a given bivalent chromosome: is any chromosome of the duplex 

 group (the two intermingled parental chromosome sets in the individual's 

 nuclei) present in the gonotokont free to pair wit h any other chromosome, 

 or does the pairing take place according to more restricting nil 



It was suggested by Henking (1891) that the two synaptic mat.- are 

 ultimately derived from the two parents at the previous fertilization, 

 one from the father and the other from the mother: the chromosomes of 

 one parental set pair with those of the other parental set to form the 

 haploid number of bivalent chromosomes appearing on the fust matura- 

 tion spindle. This idea was later emphasized and developed by Mont- 

 gomery (1900-4), Sutton (1902), Boveri (1901), and others, who found 

 for it much supporting evidence in organisms with chromosomes differ- 

 ing in size and shape. An observation made by Rosenberg L909 on 

 Drosera hybrids is significant in this connection. When Drosera rotundi- 



