264 BERTRAM G. SMITH. 



I 



between the chromosomes of a given simplex group, and by the 

 behavior of the chromosomes in synapsis. The chromosomes 

 of a single group, maternal or paternal, are not precisely alike, 

 but differ among themselves in size, form and genetic potency 

 (Montgomery, 1901; Sutton, 1902 and 1903; Morgan, 1915). In 

 certain species the size differences are very marked, so that the 

 chromosomes of a single germ-nucleus may be arranged in a 

 graded series; these size differences are constant from one cell- 

 generation to the next, so that individual chromosomes may be 

 identified in successive cell-divisions. In other species where 

 the visible differences are not so marked we have evidence that 

 physiological differences exist, for Bo'veri (1907) has shown the 

 strong probability that normal development of the egg is pos- 

 sible only in the presence of at least a single set of qualitatively 

 different chromosomes. Thus the concept of the individuality 

 of the chromosomes has been extended to include not merely the 

 genetic continuity of each particular chromosome, but also its 

 idiosyncrasy or specificity. 



In any biparental organism, the duplex chromosome group is 

 composed of two equivalent parental series or simplex groups, 

 in which each individual chromosome is homologous with a very 

 similar chromosome belonging to the other series; in other words, 

 the chromosomes are present in biparental pairs (Montgomery, 

 1901; Sutton, 1902 and 1903; Wilson, 1912). "In the process 

 known as synapsis, which takes place shortly before the last two 

 cell-divisions concerned in the formation of the germ cells, the 

 chromosomes do in fact unite in pairs, two by two. There is 

 reason to believe that the two members of each pair are respec- 

 tively of maternal and paternal derivation; and the probability 

 of this view, first stated by Montgomery, has steadily increased." 

 (Wilson, 1913). In one of these two final divisions of the germ 

 cell cycle the double chromosome groups are reduced to single 

 ones in preparation for the subsequent process of fertilization; 

 this reduction is accomplished through the failure of the indi- 

 vidual chromosomes to split in the process of mitosis. In this 

 reducing division the two members of a synaptic pair are sepa- 

 rated to pass into different daughter cells, but not necessarily 



