272 



Experimental Transjormation of Species 



MiUatiojJs 



There has now been estabhshed by means of experi- 

 ments a direct connection between the appearance and be- 

 havior of chromosomes on the one hand, and the results of 

 crossing or hybridizing on the other. This has made possible 



A 



B 



C 



2 



8 



6, 

 7 



& 

 9 



Fig. 81. The Idea of Crossing-over 



The Mendelian determiners or genes are supposed to be arranged in the 

 chromosome in a linear order, 1—9 in A. In the formation of a germ 

 cell each chromosome splits lengthwise, so that each new chromosome 

 has the same series of genes, arranged in the same order (A). In 

 fertilization the two corresponding chromosomes from the two germ 

 cells (one shown in black, the other in outline) combine so that the 

 genes unite in pairs (B). When the two corresponding chromosomes 

 become twisted during conjugation a crossing over may occur so that 

 when the members of a pair are clear of each other (C) each chromo- 

 some contains some paternal genes and some maternal ones. The break- 

 ing of a chromosome with the resulting cross-over of groups of genes 

 is more likely to occur near the middle of a series than near the end. 

 On the other hand, where two breaks occur (as suggested in the dia- 

 gram), they are more likely to take place between the middle point and 

 the ends. From Wilson, The Cell in Inheritance and Development, 

 published by The Macmillan Company. 



a clearer understanding of how new forms arise. The ex- 

 periments initiated by Mendel, and forgotten for a genera- 

 tion, were resumed in different parts of the world at the 

 beginning of this century and carried on very intensively 

 with many different species of plants and animals. In every 

 extensive study there have appeared from time to time 

 individuals having one or more distinct characters differ- 



