142 INTRODUCTION TO CYTOLOGY 



and constant pattern in a given chromosome of the complement (Fig. 79). 

 That such chromomeres are not primarily artifacts is indicated by their 

 appearance in the same configurations after a variety of fixing reagents, 

 as well as by the further fact that they may be seen in threads in unfixed 

 nuclei, especially when stretched out by the micromanipulator (Cham- 

 bers, 1924). Although their appearance may be modified in some meas- 

 ure by microtechnical treatment, they surely correspond to local differen- 

 tiations of some kind in certain regions of the threads (Agar, 1923). 



Very definite chromomeres have been described in the prophase of the 

 first meiotic mitosis in many sporocytes, notably those of liliaceous 

 plants. 1^ The significance of these bodies has been a much debated 

 question among botanists, some maintaining that they are autonomous 

 units, while opponents of such a view, especially those working on somatic 

 tissues, have suggested other explanations for the appearances observed. ^^ 

 It is true that their irregularity and indefiniteness of form in many 

 preparations suggest that they are fortuitously thickened regions of a 

 more or less homogeneous thread. On the other hand, their great 

 distinctness and regularity in other cases, notably in smear preparations 

 of Lilium and Zea, exclude so simple an interpretation. It now seems 

 evident, for reasons which will appear below, that chromomeres should be 

 regarded neither as elements wholly distinct from the rest of the thread 

 nor as mere chance accumulations of chromatic matter, but as definitely 

 localized differentiations of the chromonema, which in all probability 

 indicate the sites of certain special reactions. 



Chromomeres in the Chromonema. — In 1912 Vejdovsky, working 

 on the spermatocytes of Decticus, suggested that the chromonema is 

 made up of chromomeres which have been brought into contact. This 

 interpretation applies well to the many cases, particularly sporocytes, in 

 which chromomeres in the early prophase are reported to give way to a 

 comparatively uniform chromonema in later stages. Kuwada (1926a) 

 likens the chromomeres to "balls embedded in a rubber thread," which, 

 as it shortens, brings the balls into contact and then becomes contorted 

 within the contracting matrix. Similarly, in the sporocytes of Crepis, 

 Babcock and J. Clausen (1929) figure chromomeres in the middle prophase, 

 but at the end of the prophase only a spiral chromonema whose coils 

 may appear superficially like large chromomeres. In Lilium sporocytes, 

 fixed in chrom-aceto- formalin and stained wdth iron-brazilin, Belling 

 (19286cg, 1931a) finds the chromomeres as bead-like bodies about 0.23/x 

 in diameter and separated in the thread by regions of about twice this 



15 E.g., by Strasburger (1884, 1905 et seq.), C. E. Allen (1905), Mottier (1907), 

 Muller (1912), Belling (19286c^), and W. R. Taylor (1931). 



i«Gregoire and Wygaerts (1903), Martins Mano (1904), Gr6goire (1906 1907), 

 Marcchal (1907), Bonnevie (1908), Stomps (1910), Lundeg&rdh (1912), Sharp (1913, 

 1920), Tischler (1908, 1921), de Litardifere (19216). 



