POLYPLOID TISSUES 63 



Wlien polyploid and polysomic cells or whole individuals arise in 

 experiment they can be identified by four criteria : (i) The physio- 

 logical properties of the organism, depending on the balance of the 

 chromosomes, (ii) The number of the chromosomes, (iii) The 

 form of the chromosomes. This test is applied in the study of 

 mitosis, and is completely satisfactory in favourable material, 

 e.g., in Crepis, where each type of chromosome can be recognised 

 (Fig. 16). (iv) The pairing of the corresponding chromosomes at 

 meiosis (q.v.). 



(ii) Polyploidy. Tetraploid nuclei arise from diploid in the 

 somatic tissue of plants and animals, probably through failure of 

 the two bodies of chromosomes to separate at the anaphase of a 

 mitosis {v. somatic pairing, Ch. VI). Such nuclei are normally 

 found in certain specialised tissues of animals and very generally 

 in some Hymenoptera. They have been observed amongst 

 spermatocytes in the moth Philosamia cynthia (Dederer, 1928), and 

 in Drosophila (Bridges, 1925 a). They are also usual in certain 

 abnormal or degenerating tissues, such as the galls produced 

 by parasites {Beta maritima, Nemec, 1926) the tapetal tissue 

 surrounding the pollen mother-cells and in the embryonic mem- 

 branes of animals. Here tetraploid and octoploid nuclei are 

 regularly found. Such aberrations may also be induced by special 

 treatment, as in Boveri's experiments with echinoderm eggs, 

 where the division of the centrosome was inhibited and a single 

 nucleus reconstituted at the end of mitosis (cf. Fankhauser, 



1934). 



Particular chromosomes, especially very small ones, sometimes 



fail to divide regularly at mitosis ; both halves pass to the same 



pole, so that nuclei with variable numbers arise (Mohr, 1932), 



This has been found with small chromosomes in the extreme case in 



Tulipa galatica (Upcott, unpub.) where the same plant may have 



some nuclei with four and others with as many as nineteen small 



chromosomes. Such a wide variation can come about only where 



the small chromosomes are inert and therefore do not affect the 



growth of the cells by the changes in their number. 



The callus tissue developed round wounds by some of the higher 



plants is particularly liable to such abnormalities. Double nuclei 



