TWO CLASSES OF HYBRIDS 



First Generation Differs Widely in Character from Second and Followin,g Genera- 

 tions and the Two Classes Should Be Distinguished by More 

 Exact Names, In Order to Avoid Confusion. 



0. F. Cook 

 Bureau of Plant Industry, U. S. Department of Agriculture, Washington, D. C. 



THE use of distinctive names for 

 the two principal classes of 

 hybrids would be in the interest 

 of con\-enience and intelligi- 

 bility. The ordinal designations, "first 

 generation hybrids," "second generation 

 hybrids," "third generation hybrids," 

 etc., are cumbersome and confusing in 

 actual use. The Mendelian symbols 

 "Fi," "Fo," "F3," etc., serve for tech- 

 nical or esoteric writing, but are awk- 

 ward typographically and have little 

 meaning for the general reader. Both 

 of these s^'stems of designation are 

 essentially misleading to the student, 

 in that they leave out of account the 

 biological differences between the so- 

 called first or Fi generations of hybrids 

 and the second and later generations. 



THE NATURE OF CONJUGATION. 



Radical differences between first gen- 

 eration hybrids and later generations 

 of the same stocks seemed altogether 

 mysterious to the earlier investigators 

 of heredity, but now are looked upon as 

 necessary consequences of the special- 

 ized methods of reproduction followed 

 by the higher plants and animals. The 

 older idea of conjugation as a brief 

 period of cellular fusion applies to some 

 of the lower forms of life, but not to the 

 higher. It is only among the lower 

 organic types that the cellular body is 

 built up in the interval between the 

 completion of one conjugation and the 

 beginning of another. In the higher 

 groups the cells multiply chiefly during 

 conjugation, that is, while the cells 

 remain in a state of sexual fusion. The 

 fusion that begins with the germ-cells, 



and gives rise to the new individual, 

 does not cease when the growth of the 

 new individual begins, but continues 

 throughout its development. Beginning 

 with the subdivision of the conjugating 

 germ-cells, all of the cells that form the 

 bodies of the higher plants and animals 

 are double, with respect to their nuclear 

 elements. Either there are two sep- 

 arate nuclei in each cell, or the two 

 parental sets of chromosomes remain 

 separate inside of the nuclei. The con- 

 cluding stage of conjugation is mitapsis 

 or fusion of the chromatin material, 

 which finally arranges itself in the form 

 of two long parallel threads. Conjuga- 

 tion is not concluded in the somatic 

 cells, but only in the reproductive cells, 

 as a preliminary to the formation of the 

 next generation of germ-cells.' 



NAMES OF THE CLASSES. 



The usual object of experiments with 

 hybrids is to analyze and recombine the 

 characters of the parental types, and 

 for this purpose at least two generations 

 must be produced. As the so-called 

 first generation of a hybrid is developed 

 while the conjugation begun by the 

 parental germ-cells is still in progress, 

 it can be described as the conjugate 

 generation. The so-called second or F2 

 generation is really the first generation 

 that can be considered as a complete 

 product of the conjugation that was 

 begun by the original germ-cells. The 

 name perjugate seems appropriate be- 

 cause the nuclear elements represented 

 in the second and later generations of a 

 hybrid may be said to have passed 

 through conjugation. Conjugate means 



1 Cook, O. F., and Swingle, W. T., Evolution of Cellular Structures, U. S. Department of 

 Agriculture, Bureau of Plant Industry, Bulletin 81, 1905. See also, Proc. Washington Academy 

 of Sciences, Vol. IX, pp. 191-197, 1907. 



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