V TETRAPLOIDY 



149 



^P^°^^^* Number of Chromosomes 



Ascaris megalocephala univalens 



,, ,, hivalens 



Cypris reptans, parthenogenetic . 



fuscata, 

 Ophryotrocha puerilis, some individuals 



other 

 Oenothera lamarckiana .... 



gigas 



Musa sapientum (the banana), one variety 

 „ ,, another variety 



(Diploid). 

 \ Many authors. 



2 j Schleip, 1909. 



g| Korschelt 1895. 



14 Various authors. 



28 Gates, etc., 1909 a. 



i6| 



32 ;- Tischler, 1908. 



48 



In a species in which distinct size differences exist between the 

 different chromosomes, the question of tetraploidy should be capable of 

 easy solution, since in this case the chromosomes should be grouped 

 according to sizes in fours instead of in pairs. Practically nothing seems 

 to be known about this matter in supposed tetraploid forms. Montgomery 

 (1909), however, has described the chromosomes of the hivalens variety 

 of^. megalocephala as consisting of a longer and a shorter pair. The 

 difference between the pairs is indeed slight, but if established it 

 would make it probable that this form is not tetraploid. This case is 

 especially instructive, as here if anywhere one might have expected 

 the four chromosomes to have been derived by doubling of the two 

 chromosomes of univalens, since the total volume of the four chromosomes 

 in hivalens is even more than double that of the two in univalens (Brauer, 



1893) 



Two genera of the Oligochaete family Enchytraeidae present similar 

 relations (Vejdovsky, 1907). In Fredericia hegemon 2^=32, and in 

 Enchytraeus humiciiltor 2^ = 64. Nevertheless, the chromosomes in the 

 latter genus are much longer and thicker than in the former. 



In certain cases, however, especially those in which the forms with 

 doubled chromosome number have arisen under experimental conditions, 

 the comparison of the volumes of the chromosomes is of value in deciding 

 how the doubling has occurred. Two such cases are found in the plant 

 genera Oenothera and Primula. 



Oenothera gigas is one of the well-known " mutants " of 0. lamarckiana. 

 It first appeared as a single individual in de Vries' cultures of i8g8, and 

 in the next twenty years the appearance of six more individuals was 

 recorded from the cultures of de Vries and others, so that its origin from 

 the parent form was observed altogether seven times in that period. 

 Once arisen, it breeds pure to its peculiar characteristics (except for a 

 tendency to give off exceptional mutants of the same order as those 

 produced by the parent species). These characteristics are the possession 

 of 28 chromosomes in place of 14, and the greater size of nearly all its 

 parts, such as stalk, leaves, petals, etc. 



Gates (1909 a) has compared the relative sizes of the cells in 0. gigas 



