VI 



FUNCTION OF SYNDESIS 



175 



During their apposition exchange of corresponding factors takes place, 

 and the chromosomes after separation may be constituted in various 

 ways, e.g., AhCdE and aBcDe ; or abCDe and ABcdE, etc., etc. 



By this means the Mendehan inhoritance of any nunil^cr of «;opnrntr 

 characters can be accounted for. 



The necessity of assuming the intcrchangcabihty between homologous 

 chromosomes of the chromosome components makes it highly desirable 

 to determine whether parasyndesis is or is not of general occurrence. 

 Parasyndesis obviously 

 offers a favourable op- 

 portunity for the mutual 

 exchange between con- 

 jugating chromosomes 

 of their elements which, 

 as we have seen, are 

 arranged in linear series 

 — and indeed at this 

 stage the corresponding 

 chromomeres of the two 

 chromosomes are often 

 most regularly and con- 

 spicuously in close ap- 

 position (Fig. 77). The 

 evidence for parasyn- 

 desis was discussed in 

 Chapter II., and its 

 general occurrence 

 provisionally accepted. 



\A7p h^\T(^ mnrpnver Examples of the correspondence between th.- chroniornon-s m hoino- 

 VVe nave, lliuicuvt,i, Jq^^us chromosomes during syndesis. A, Spttutx niger i ; H. .^/>l.»w 

 ^• j. -.pj,c:p^^ fp,^ KpljpV- glutinosa i. (A and B, after Schreiners, ,4.fi.. igotj) C, Dytiuus mar- 

 aireCt reason lOr Oenev |^.^^^.^^ (after Henderson, Z.u-.Z., 1907). 



ing that the function of 



syndesis is not merely that of bringing the members of homologous pairs 

 'into apposition so as to effect their sorting out into different daughter 

 nuclei at meiosis. For syndesis often takes place months or years 

 (mammahan oocytes) before the reduction division, and is frequently 

 followed by complete separation of the ex-syndetic chromosomes (many 

 oogeneses, spermatogenesis of Lepidosiren, etc.). Between syndesis and 

 metaphase I. the chromosomes may even undergo metamorphoses 

 as great as those undergone in the resting nucleus (most cases 

 of oogenesis). The separated homologous chromosomes ilun pair 

 again in prophase I. immediately before the metaphase (oogenesis, 

 Lepidosiren spermatogenesis). This second pairing, which is not of the 



