SPECIAL PROBLEMS OF MITOSIS 29 



The two chromosomes of a pair are said to be 

 homologous, since they contain the same series of 

 genes arranged in the same order. The concept of 

 homology is one which may be appHed to parts of 

 chromosomes as well as to whole ones, since one some- 

 times finds a pair of chromosomes which are homo- 

 logous in some regions but not in others (see below). 



In some organisms the chromosomes can be grouped, 

 not into pairs, but into threes, fours or groupings of 

 higher numbers. Such organisms (in which the soma- 

 tic number is not diploid) are called polyploids^ those 

 with three of each kind of chromosome being triploidSy 

 those with four tetraploids and so on (pentaploids, 

 hexaploids, heptaploids, octoploids, &c.). 



The lowest diploid number found in any organism 

 is 2, which occurs in the Roundworm, Ascaris megalo- 

 cephala var. univalens (this species also has a tetra- 

 ploid variety, bivalens with 4 chromosomes in the 

 diploid set).* The highest diploid numbers hitherto 

 recorded are 208 for a Crayfish ^^ and a Crab ^^ 

 and 200 for the Great Water Dock (Rumex hydro- 



* These numbers for the two varieties of Ascaris megalo- 

 cephala refer only to the germinal tissues. In the somatic 

 tissues a much larger number are found, as a result of frag- 

 mentation of the 2 or 4 originally present in the fertilized 

 egg (see later). 



Fig. 5. — Somatic chromosome sets of various organisms. 

 a after Making, i®^ b after Darlington,^* d and e after 

 Yamamoto, / and g after Hughes Schrader,'** i after 

 Matsuura and Suto (J. Fac. Set., Hokkaido Imp. Univ. 

 Ser., V, 5, 33), ; after Morgan, 127 the rest original. 

 All figures slightly re- drawn, b, d, c, i are plants, the 

 other organisms are animals. In h the coiling of 

 the chromatids round one another is noticeable, in i 

 the chromatids are parallel. The spindle attachments 

 cannot be seen in c, /, and g, but are visible in all the 

 others. In o, c, /, g, and j no trace of a ' split ' between 

 the chromatids can be seen, in the others it can be seen, 

 either at the ends of the chromosomes, or throughout. 



