EXTRA CHROMOSOMES — BL.\KESLEE 433 



IoIlI, or 111 stage tliere is only a single chromosome in each set, as 

 shown in the diagram. According to the conventional use, a circle 

 represents the female sex cell and a s<iuare the male sex cell. After 

 pollination, when the pollen tubes have grown down to the ovary and 

 have let out the male sex cells, fertilization takes place by the union 

 of the latter with the egg cells. The result of fertilization is a seed 

 in which the number of chromosomes is doubled. We come again, 

 therefore, to the 2n or diploid stage. 



All the higher })lants and animals show this distinction between 

 u lin stage, in which there are two chromosomes in each set; and 

 a In stage, including the sex cells, in which there is only one chromo- 

 some in each set. The number of sets, however, varies widely among 

 the dirt'erent species, although constant for any given form. Thus 

 in the fruit fly there are 4 chromosomal sets, in man 24 sets, in the 

 jiightshade 36 sets, and in the Jimson weed 12 sets. In many spe- 

 cies there is no visible dillerence between the individual chromo- 

 somes, although the breeding evidence may prove that they carry 

 ditferent factors. In the Jimson weed, however, size classes can 

 be distinguished, as shown diagianuuatically in the figures. Some 

 of the chromosomes are further distinguished by presence of humps. 

 It should be pointed out that, although the diagrams show roughly 

 the relative sizes of the cliromosomes, the arrangement is purely 

 diagrammatic. The point to be emphasized is the grouping of the 

 chromosomes into 12 separate sets although their association into 

 such sets can be seen only in the cells immediately preceding 

 the reduction division. 



ABNORMAL ARRANGEMENT OF CHROMOSOMES 



While no change in the number of 12 chromosomal sets is known 

 to have taken place in the Jimson weed, a considerable variation in 

 the number of chromosomes within the sets may occur. Some of the 

 chromosomal types that have been identified are shown diagram- 

 matically in Figure 2. Since these diagrams represent the condition 

 in the adult plant, only the diploid (2n) condition is normal. The 

 plants are classified into balanced and unbalanced types according 

 to whether or not all the sets have the same number of chromosomes. 



BALANCED CHROMOSOMAL TYPES 



Balanced types may be still further divided into even-balaiu cd or 

 rrtable. and odd-balanced or unstable types. The former have an 

 even number of chromosomes in each set. form sex cells with half the 

 /lumber of chromosomes, and hence breed essentially true. The 

 latter have an odd number in a set, and are, therefore, unable to 

 form sex cells with an equal division of chromosomes, and hence can 

 not breed true. 



