272 INTRODUCTION TO CYTOLOGY 



It may be added that when there are several chiasmata in a tetrad the 

 same two chromatids are not necessarily involved at all of them. This 

 creates a more complicated situation, but it should not be allowed to 

 obscure the principle just illustrated with a simpler tetrad. The state- 

 ment emphasized above still holds for the region about any one chiasma 

 in a more complex tetrad. 



It should now be clear why, as a general rule, neither of the two meiotic 

 divisions is wholly disjunctional or wholly equational in character. Even 

 if it be held that the first mitosis is regularly disjunctional for the spindle- 

 attachment regions of the chromosomes, it is evident that this mitosis 

 must be equational for certain other regions when chiasmata and chroma- 

 tid exchange are involved. The conclusions stated at page 255 should 

 now be reread in the light of the foregoing discussion. 



The Synaptic Reaction. — The forces bringing about synapsis ordi- 

 narily begin to manifest themselves in the early prophase of the first 

 meiotic mitosis and operate for a comparatively brief period. In certain 

 exceptional cases pairing is evident during the closing phases of the last 

 premeiotic mitosis and may become so intimate that it looks much like 

 synapsis. ^^ Pairing has sometimes been reported in the spermatogonia 

 several cell generations before meiosis.'^ Again, cases are known in 

 which a tendency to associate loosely in pairs is shown in some measure 

 in the somatic cells. In extreme examples the pairing occurs directly 

 after the gametic chromosome sets are brought together in syngamy and 

 persists throughout development. This is notably true of the Diptera, 

 as shown especially by Metz (1916a et seq.) (Figs. 71, 196). A tendency 

 toward such a paired arrangement, but no actual synapsis, has frequently 

 been reported in somatic cells of plants. Reports of a "late synapsis" 

 taking place at diakinesis are frequent in the literature. It has been 

 shown in some such cases^^ that this is probably "a secondary coupling 

 that has been preceded by a typical synapsis at the usual time and a 

 subsequent deconjugation" (Wilson). 



The two chromosomes which undergo synapsis to form each tetrad are 

 the corresponding members of the two monoploid sets composing the 

 diploid complement. They are "homologous" not only because of their 

 ultimate common origin but also in the sense that they perform similar 

 roles in the life of the organism. More specifically stated, they somehow 

 influence similar groups of functional reactions during the development 



^^ Asilus (Metz and Nonidez, 1921), Drosophila (Metz, 19266). See also Mont- 

 gomery (1901) on certain Hemiptera, Sutton (1902) on Brachystola, M. Blackman 

 (1903, 1905) on Scolopendra, and Dublin (1905) on Pedicellina. 



32 Certain Hemiptera and Ascaris (Montgomery, 1904, 1905, 1908, 1910), Alytes 

 (Janssens and Willems, 1909), Helix and Sagiita (Stevens, 1903; Ancel, 1903), certain 

 Diptera (Stevens, 1908, 1911), and Pediculus (Doncaster, 19206). 



^^ Lepidosiren (Agar, 1911), Rhodites (Hogben, 1920a), Cyanotis (Rau, 1930). 

 See Wilson (1925, p. 563). 



