62 THE CHROMOSOMES 



This process is called pairing or (in the older accounts) 

 synapsis. In the case of each pair of homologous 

 chromosomes the pairing process begins at one or 

 more points and then spreads along the length of the 

 chromosomes (Fig. 96). Where the telophase arrange- 

 ment of the previous division has been retained 

 pairing begins at the spindle attachments — other- 

 wise it may T^egin anywhere. It must be pointed out 

 that the pairing is not merely between homologous 

 chromosomes, but always between strictly homo- 

 logous regions ; this can be seen very clearly where 

 the chromomeres are distinct, since they are of 

 slightly different sizes (Fig. 96). If we call those in 

 one homologous chromosome a, b, c, d, . , . and 

 those in the other a', b', c', d', . . . then a will pair 

 with a' and b with b' and so on. If a short region 

 has become inverted in one homologous chromosome 

 but not in the other (as sometimes happens) then the 

 inverted region will remain as an unimired loop in the 

 middle (Fig. Ha). If a rather longer section is 

 inverted the loop will twist round and pair as in 

 Fig. 116. If a small section is completely missing 

 from one chromosome, then the corresponding section 

 in the homologous chromosome will form a short 

 loop (Fig. llc).^^ It appears, therefore, that the 

 force of attraction is a mutual one between homo- 

 logous chromomeres (or genes) and that it is probably 

 identical with the force which keeps the two chroma- 

 tids of a chromosome together throughout their length 

 in the prophase of mitosis. It seems natural that this 

 force should lead to a pairing of homologous chromo- 

 somes at zygotene, since these have not undergone 

 the usual longitudinal division — they are still unsplit 

 at a stage when they would normally be split ; the 

 force of attraction thus satisfies itself at mitosis by 

 maintaining chromatids together and at zygotene 

 by bringing distinct chromosomes into longitudinal 

 approximation. 1^* Darlington regards the pro- 

 phase of the first meiotic division as ' preqocious ' in 



