THE REDUCTION OF THE CHROMOSOMES 255 



It should be pointed out that cytologists and geneticists have generally 

 assumed that each synaptic pair is independent of all the others as 

 regards the manner in which it is oriented on the heterotypic spindle. 

 In some pairs the paternal members are directed toward one pole and in 

 other pairs toward the other pole. It is conceivable that in some cases 

 all the paternal members might go to one pole and all the maternal 

 members to the other. Direct evidence that the assortment of the 

 various chromosome pairs is in this respect a random one as originally 

 assumed has been furnished by Miss Carothers (1913, 1917). In the 

 grasshopper, Trimerotropis, she finds that the components of some of the 

 bivalents are visibly different in size, in the mode of attachment to the 

 spindle fibers, and in the presence of constrictions; and that these differ- 

 ences make it possible to show beyond question that the several pairs are 

 entirely independent of one another as regards their orientation on the 

 spindle and their consequent distribution to the daughter cells. 



From the precise manner in which the distribution of chromosomes 

 at the time of reduction and at other stages of the life cycle parallels the 

 distribution of the hereditary characters it is inferred that such hom- 

 ologous chromosome pairs represent the material basis for the allelo- 

 morphic pairs of Mendelian characters exhibited by the organism. This 

 subject is to be taken up in Chapter XV. 



The Stage at Which Conjugation Occurs. In the great majority of 

 observed cases chromosome conjugation occurs during the prophase 

 of the first maturation division. Since the chromatin threads at some 

 time during these prophases usually take the form of a tightly contracted 

 knot out of which they emerge in an obviously double condition, it was 

 suggested (Moore 1896) that the contraction is an important factor in 

 bringing about the conjugation, and the contraction itself came to be 

 called " synapsis." But an examination of the various modes of reduction 

 shows that the conjugation may begin very early, before the contraction 

 (Fig. 83) or, on the other hand, not until the spindle is established 

 (Fig. 95, /)). The conjugation of the chromosomes is therefore to be 

 distinguished from the contraction. It has now become customary to 

 refer to the former, at whatever stage it occurs, as synapsis, and to the 

 latter as synizesis. 



In an increasing number of reported cases the paired association 

 apparently begins even before the heterotypic prophase. The chromo- 

 somes have been observed in several instances to undergo pairing during 

 the anaphase and telophase of the last premeiotic division. Such is the 

 condition in certain Hemiptera (Montgomery 1900, 1901), Oniscus 

 (Nichols 1902), Brachystola (Sutton 1902), Scolopendra (Blackman 

 1903, 1905), Pedicellina (Dublin 1905), and a number of more recent 

 cases. Furthermore, the pairing has been stated to begin in the sperm- 

 atogonia several cell generations before maturation in certain Hemiptera 



