266 INTRODUCTION TO CYTOLOGY 



several tetrads in one mitotic figure may behave differently in this respect, 

 each of the two mitoses in such cases may be disjunctional for some 

 elements and equational for others. Taking the chromosome comple- 

 ment as a whole, the disjunction of its homologous elements and the 

 equational separation of their halves are complete only at the conclusion 

 of the second mitosis. 



As will be shown in subsequent chapters, genetic data yield valuable 

 evidence bearing on the distribution of chromosomes in meiosis. By such 

 methods it can be shown, especially when the four products of meiosis can 

 be followed individually,^^ that some pairs of genetic units segregate in / 

 while others do so in //. There is evidence which indicates that, although 

 some portions of a tetrad divide equationally in /, the division tends 

 strongly to be disjunctional in the neighborhood of the spindle-attach- 

 ment region." How such a result could be brought about by an exchange 

 of portions between two non-sister chromatids in the tetrad will be shown 

 in the following section on the chiasma. Certain observations suggest 

 either that sister chromatids in the tetrad are more closely associated 

 at the attachment region than are non-sisters, possibly because this region 

 divides very late, or that for some other reason the non-sisters {i.e., the 

 synaptic mates) tend to separate rather consistently in 7. To what 

 extent this suggestion can be generalized and applied to the markedly 

 heteromorphic tetrads mentioned above cannot be stated at present. 



The Chiasma. — It was once thought by some observers^* that the 

 two homologous chromosomes entering into synaptic union fused com- 

 pletely and then split twice along new planes to form the four chromatids 

 of the tetrad. Subsequent investigation has shown this to be erroneous, 

 as was claimed from the first by another group of workers. ^^ Modern 

 cytologists and geneticists are now generally agreed that chromosomes 

 preserve their essential identity through the period of synaptic associa- 

 tion, at least so far as concerns those chromonematic elements which are 

 responsible for their finer morphology and genetic functions. 



This statement requires one very important qualification. It is 

 now known that frequently two of the chromatids in the tetrad exchange 

 corresponding portions with each other, as originally maintained by 

 Janssens (1909). Beginning with Morgan (1911a) this "crossing-over" 

 has been regarded as the mechanism responsible for the frequent recom- 



^^ E.g., the four spores of a single quartet in Sphoerocarpos (C. E. Allen, 1924 et 

 seq.; see 1930a). 



" Morgan (1925), Bridges and Anderson (1925), Anderson (1925, 1929), Redfield 

 (1930). 



^^E.g., Vejdowsky (1907), Bonnevie (1906, 1908, 1911), von Winiwarter and 

 Sainmont (1909), and H. Schneider (1914). 



" Berghs (1904, 1905), A. and K. E. Schreiner (1905, 1906), Marcchal (1907), 

 Gregoire (1907, 1910), Schleip (1906, 1907), Montgomery (1911), J. B. Overton 

 (1905, 1909), Robertson (1915, 1916), Kornhauser (1914, 1915), Wenrich (1915, 1917). 



