May, 1915.] 



The Chromosome Mechanism. 



5" 



race pure in respect to all hereditary phenomena resulting from 

 the activity of the pair, or the pair may be heterozygous in which 

 case Mendelian phenomena must result. 



Now it will be apparent that with a definite number of chromo- 

 somes whose activities during the life cycle are known it can be 

 determined before hand just what segregations and combinations 

 of hereditary factors are possible. If the chromosomes are the 

 only bearers of heredity, there should not be more Mendelian 

 segregations of two absolute, heterozygous hybrids than the per- 

 mutations possible with the number of chromosomes. By an 

 absolute, heterozygous hybrid is meant one in which all the uni- 

 valent chromosomes have at least one distinctive factor. So 



a 



s 



Fig. 1, a, b, c. Bivalent or reduction chromosomes from a megasporo- 

 cyte of Lilium philadelphicum. The two longitudinal limbs of the twisted 

 loop represent two univalent chromosomes, one maternal and one paternal, 

 fused end to end in synapsis and folded lengthwise, the synaptic joint being 

 at the head of the loop. Each univalent has already divided longitudinally 

 into two daughter chromosomes but these are not evident in the figures 

 which were taken from a preparation stained with a rather diffuse stain. 

 It was this type of chromosome which first lead the writer to the conclusion 

 that the reduction division is a qualitative division. The true nature of 

 the formation and division of these chromosomes can only be determined by 

 studying the preceding and subsequent stages. 



far as the writer knows, the possible segregations of distinct com- 

 binations have never been tested practically. In Canna (as will 

 appear below) which is said to have but six univalent and three 

 bivalent chromosomes, there would be twenty-seven possible 

 varieties from two original pure lines without considering possible 

 new characters which might appear as the result of the activity 

 of a heterozygous pair. This is perhaps the best plant on which 

 the theoretical expectation might be tested out. Unfortunately 

 many of the varities produce little or no seed. The hybridization 

 would have to be carried on between varieties giving completely 

 fertile offspring. 



