134 CYTOLOGY chap. 



male parent (Fig. 63). This number could be counted (in the germ 

 track) up to at least the 12-cell stage, which was the latest stage examined. 

 This case must not be confused with the A. megalocephala with three 

 chromosomes described by Boveri (p. 145), though the two cases furnish 

 equally strong evidence for the continuity of the chromosomes. 



B. THE COMPOSITION OF THE CHROMOSOMES 

 OF SMALLER UNITS 



While the definitive chromosomes of the metaphase generally appear 

 homogeneous, they characteristically present a different appearance in 

 prophase, where they are often markedly moniUform, i.e. consisting of 

 a row of bead-hke sweUings of chromatin, called chromomeres, joined 

 to each other by a thinner hnin thread. This condition, which forms 

 one of the most characteristic sights met with by the cytologist, can 

 be illustrated by reference to almost any work deaUng with mitosis, 

 whether in the soma, germ track or during meiosis. Many figures in 

 this book illustrate this point incidentally {e.g. Figs. 20, yy). It is 

 equally characteristic of animals and plants (Fig. 64). 



In the early prophase the chromomeres, if visible, are commonly 

 very small and numerous (when not visible it probably means that they 

 are so closely distributed along the chromosome that their boundaries 

 are not distinct). As prophase proceeds they become larger and fewer, 

 obviously by fusion in groups. At about this stage they are often 

 extremely prominent, constituting comparatively large spheroidal 

 swellings joined to their neighbours by short stretches of very fine threads. 

 As the chromosomes contract, the now composite chromomeres become 

 more and more pressed together, the boundaries between them gradually 

 becoming obhterated till in the metaphase chromosome they are generally 

 no longer distinguishable from one another, and in consequence the 

 chromosomes appear homogeneous. Finally, in the greatly contracted 

 chromosomes of meiosis, or somatic telophase, the chromomeres appear 

 to lose their Hnear arrangement. We must however suppose that the 

 loss of the linear arrangement is only apparent, and that essentially it 

 is maii)tained so that the chromomeres appear in the same order in the 

 prophase chromosomes of successive mitoses. 



In the early prophase the chromomeres are often the points of de- 

 parture for the hnin threads which run out froQi the chromosomes into 

 the vanishing nuclear- reticulum {e.g. Figs. 3, 16). This suggests that the 

 substance forming the chromomere has travelled down the hnin fibre to 

 the main trunk of the chromosome. 



The correspondence, as regards number and sizes of the chromomeres, 

 between the daughter threads of the spht somatic chromosome (Fig. 



