6. Vererbungslehre. 243 



delian segregation and recombination are represented in the fertilized egg by 

 cell organs, the chromosomes, or some other organs which behave as the 

 chromosomes behave in the reduction division. It is not assumed that each 

 heritable character is represented in the germ cell by a specific structural 

 element but that the cell organs possess properties which in a measure pre- 

 determine development. In other words, each of these individual cell organs 

 possess specific properties by virtue of which, under appropriate environment, 

 and at an appropriate stage of the developmental process, it is able to affect 

 the further course of development. This theory does not assume that a Single 

 cbromosome or other cell organs is wholly responsible for the development 

 of any organ or region of the body. On the other hand a Single cell organ 

 may have a determining influence in the development of more than one heri- 

 table character. M. E. Curtis (Orono). 



671) Joliannsen, W., The genotype conception of heredity. 



(Amer. Naturalist 45. p. 129—159. 1911.) 



In this important paper the author distinguishes sharply between the 

 transmission-conception and the genotype-conception of heredity, emplasizing 

 the fact that the personal qualities of an individual result from "the reactions 

 of the gametes joining to form a zygote; but the nature of the gametes is not 

 determined by the personal qualities of the parents or ancestors in question". 

 The accuracy of the conceptions resulting from the Mendelian and pure line 

 work is contrasted with the Lamarckian and biometrical conceptions, which 

 involve the idea of "transmission" of the personal qualities of the ancestors. 

 Heredity is defined as "the presence of identical genes in ancestors and des- 

 cendants", and a pure line is "the descendants from one Single homozygotic 

 organism, exclusively propagating by self-fertilization". 



It is believed that selection is not able to shift the nature of the geno- 

 type — a conclusion based on the work of Jennings, Woltereck, Pearl 

 and others. It is held, furthermore, that there is discontinuity between 

 different genotypes, just as between Mendelian factors. Two genotypes or 

 „Reaktionsnorms" can only be compared by growing the organisms under the 

 same conditions. If they react so as to produce the same race then both 

 belong to the same genotype, but if grown in varying conditions the confluence 

 of their curves does not prove that they belong to one genotype. Various 

 other features of the genotype hypothesis are discussed. Gates (St. Louis). 



672) East, E. M., The genotype hypothesis and hybridization. 



(Amer. Naturalist 45. p. 1G0— 174. 1911.) 



The writer discusses certain features of Mendelian inheritance in rela- 

 tion to the genotype hypothesis, and believes that the two are interdependent. 

 He produces evidence to show that in certain maize crosses, size characters 

 segregate in the F 2 . A medium-sized flint maize crossed with a tall dent 

 maize gave an F x nearly as tall as the tall parent. This result is not 

 ascribed to dominance, but to the increased vigor that comes from crossing. 

 In the F 2 the destribution of heights is much more variable than in Fj, the 

 coefficient of Variation averaging 13,44 as against 8,68 in Ft.. A dwarf pop 

 maize crossed with a tall sugar com gave similar, though less marked results. 



The same segregation was found with regard to length of ears and 

 weights of seeds in the latter cross. From the number of extreme segre- 

 gates obtained, it is considered probable that such a character as "size of ear" 

 in these crosses is due to not less than three or four independent interacting 

 characters. Gates (St. Louis). 



