140 INTRODUCTION TO CYTOLOGY 



medium. ^1 Certain studies on the chemical composition of plant 

 chromosomes suggest that the chromonemata consist of nucleo-protein 

 and probably lipide, and the matrix of lipides free or combined with 

 protein (Shinke and Shigenaga, 1933). 



In many plants used in such investigations the chromonema appears 

 in late prophase, metaphase, and anaphase as a rather smooth thread 

 coiled with striking regularity within the matrix. The clearest prepara- 

 tions show it to be double. In other plants the chromosomes in the 

 sporocytes do not show such regularly coiled and smooth chromonemata 

 as those described above. In Zea Mays, for example, the shortened 

 chromosome in these stages presents an appearance rather similar to 

 that observed in the earlier phases. The chromonemata, which show 

 their chromomeres clearly, seem to shorten with the chromosome as a 

 whole and preserve much of their early structure without being thrown 

 into very wide coils. This point will be considered further in a subse- 

 quent section. The behavior of the chromonemata through the mitoses 

 in the sporocyte will be discussed in Chapter XVI. 



^ 



Fig. 78. — Chromosomes in metaphase of first meiotic mitosis, showing aspects observed 

 after different fixations. (After Kaufmann, 1926a.) 



The Alveolation Theory. — According to this theory of chromosome 

 structure, which was founded chiefly on studies of somatic cells in plants, ^^ 

 the more or less homogeneous chromosomes undergo a progressive 

 "alveolation" during the telophase. Lightly staining regions appear in 

 irregular positions in the chromosomes, whose chromatic matter assumes 

 a spongy structure and forms the reticulum of the interphase and meta- 

 bolic stage. In the ensuing prophase the reticulum again breaks up into 

 its constituent "elementary nets," each of which gradually transforms 

 into a slender filament. This filament splits, shortens, and thickens to 

 become the double chromosome whose halves pass to the poles in the 

 anaphase. The slender chromatic thread is accordingly looked upon as a 

 temporary formation of the prophase related to splitting rather than a 

 structure persisting throughout the nuclear cycle. 



It is true that such figures as those published by the advocates of this 



theory represent rather accurately the aspects often observed in the 



telophase and early prophase; but it is now held by various workers, 



including some who formerly supported the alveolation theory, that they 



should be looked upon as inadequate images of the chromonemata and 



matrix during these stages (Fig. 78). The "alveolation" is therefore 



" Kuwada and Sakamura (1926), Sakamura (1927a). 



12 Gr6goire and Wygaerts (1903), Gregoire (1906), Lundegardh (1910, 1912), Sharp 

 (1913, 1920a), de Litardiere (19216), J. B. Overton (1911, 1922), Koerperich (1930). 



