THE PHYSICAL BASIS OF MENDELISM 63 
chromosomes. In the mitoses which have preceded this particular divi- 
sion, the chromosomes were each time conceived to be reformed from the 
identical group of chromomeres which they contained originally. In 
synapsis, however, as shown at B there is a certain amount of intertwin- 
ing of the paired threads and-in the unraveling of the chromosomes after 
the contraction stage there is likewise a twisting of the filaments about 
each other. The indications are, therefore, that in synapsis there is a 
possibility of interchange of chromatin material between the members 
of a pair of homologous chromosomes. In all cases, however, in order 
to uphold our conception of the definite organization of the chromosomes 
with respect to the chromomeres which they contain, this interchange of 
material must involve exactly equivalent portions of the two chromo- 
fel 
Fia. 30.—Diagram of chromatin interchange between homologous members of a pair of 
chromosomes. (After Muller.) 
somes. The chromosomes of the reduction division shown at F may not, 
therefore, be identical with the four originally present in A, but may 
represent various combinations of portions of both members of a par- 
ticular pair of chromosomes. >The results of such interchange between 
members of homologous pairs of chromosomes is shown in Fig. 30. At 
the left is shown a pair of chromosomes one in outline the other in full 
black. In the middle the steps in chromatin interchange are diagrammed 
and finally at the right this interchange results in a pair of chromosomes 
each of which is made up of parts of both members of the original pair 
of chromosomes. Various combinations may result depending on the 
points at which interchange takes place, but in every case the exchange 
involves corresponding portions of the two chromosomes. 
Independent Distribution of Chromosomes.—In Fig. 31 are illus- 
trated diagrammatically the chromosomes of Drosophila, with particular 
reference to their size and form relations and to their characteristic 
pairing in the cell. One member of each of these pairs of chromosomes 
was contributed by the female parent and one member by the male parent. 
In the reduction divisions these chromosomes are separated so that 
each germ cell contains one member of each pair of chromosomes. The 
simplest condition which could obtain is that of independent distribu- 
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