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., AbCdE and aBcDe ; or abCDe and ABcdE, etc., etc. 



By this means the Mendelian inheritance of any number of separate 

 characters can be accounted for. 



The necessity of assuming the interchangeability 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. 



We have, moreover, 

 direct reason for believ- 

 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 

 (mammalian 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 then pair 

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

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



Examples of the correspondence between the chromomeres in homo- 

 logous chromosomes during syndesis. A, Spinax niger ; B, Myxine 

 glutinosa <5- (A and B, after Schreiners, A.B., 1906.) C, Dytiscus mat - 

 ginalis (after Henderson, Z.w.Z., 1907). 



