THE PHYSICAL BASIS OF MENDEL1SM 



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- 



Fio. 30.- 



-Diagram of chromatin interchange between homologous members of a pair of 

 chromosomes. (After Mutter.) 



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- 



