MEIOSIS 273 



of the organism and presumably after maturity. Their effects on the 

 course of development may or may not be precisely the same, but they 

 differ regularly in certain important respects from those exerted by the 

 other chromosomes of the complement. As a consequence of this influ- 

 ence on developmental reactions, the two homologous chromosomes are 

 responsible in some measure for the appearance of morphological and 

 physiological characters which are the results of those reactions. Very 

 little is known concerning the nature of the physical and chemical reac- 

 tions through which the chromosomes thus influence development, and 

 still less can be said about additional roles they presumably play in the 

 metabolic activity of the mature organism. It will be seen in the follow- 

 ing chapter how researches in the field of cytogenetics have established 

 firmly the theory that a given chromosome influences the development of a 

 certain group of characters because it is composed in part of a certain 

 number of semiindependent elements known as genes. In the terms of 

 genetics, therefore, homologous chromosomes carry similar groups of 

 genes. 



Ordinarily the two members of a homologous pair are about alike in 

 size and form, so that in many diploid chromosome complements they 

 can be readily recognized on this basis (Figs. 67, 69). Sometimes they 

 exhibit certain differences, some of which can be shown to bear a relation 

 to differences in function. An extreme case of this is seen in the sex- 

 chromosome pair in many animals and plants. Moreover, non-homol- 

 ogous chromosomes are often alike so far as the eye can tell. Hence 

 it becomes unsafe to depend upon size and form alone as criteria of 

 homology. 



The most reliable cytological criterion of homology is normal synapsis 

 itself. Caution is nevertheless necessary in using it, for it has been 

 found that a number of agencies may induce a partial or complete failure 

 of synapsis (asynapsis) on the part of chromosomes known to be homol- 

 ogous.^'* The chromosomes may sometimes synapse in the megasporo- 

 cytes but not in the microsporocytes. Cases are even known in which 

 asynapsis behaves as a recessive Mendelian character. It should be 

 pointed out that absence of pairing in late prophase and metaphase alone 

 should not be accepted unconditionally as evidence of asynapsis, for the 

 chromosomes may sometimes synapse as usual and then separate before 

 the end of the prophase. Such absence of pairing may therefore indicate 

 desynapsis ("deconjugation") rather than true asynapsis (" non-con juga- 



^^ E.g., Abnormal temperatures (Belling, 1925a, on Uvularia; Sakamura and Stow, 

 1926, on Gagea; Stow, 1926, 1927, on Solarium; and others); a recessive Mendelian 

 gene (Beadle and McClintock, 1928, and Beadle, 1930a, on Zea; Schwemmle, 1928, 

 on (Enoihera); X-rays (Goodspeed, 19296, on Nicoiiana); nutritional differences in 

 anthers and ovules (J. Clausen, 19306, on Viola); genetical conditions affecting pairing 

 in hybrids (Kihara, 1929c; Karpechenko, 1930). See further footnotes in Chap. XIII. 

 Synapsis is often deficient in plants with more than two homologues (p. 355). 



