i THE CHROMOSOMES 13 



to the continuity of the chromosomes, this double series is perpetuated 

 throughout the nuclei of the growing embryo. Corresponding chromo- 

 somes, for example, the two A's, are known as homologous chromosomes. 



The above remarks on the continuity of the chromosomes and the 

 nature of the chromosome equipment of organisms are anticipatory 

 of the later chapters of the book, where these points will receive more 

 detailed consideration ; we will now return to certain problems of mitosis, 

 the main steps of which we have just outlined. 



(i) The Division of the Chromosomes, and their Relation to the Resting 



Nucleus 



As we have seen, the prophase chromosomes are often from their 

 first appearance double, i.e. split into two daughter chromosomes, 

 possibly signifying that the chromosomes, or rather the elements of 

 which they are composed, were already divided in the resting nucleus. 

 It is even possible that the actual moment of division may be during the 

 anaphase of the previous mitosis. 



The problem of the mode and moment of division of the chromosomes 

 is therefore intimately bound up with the question of the exact processes 

 by which the compact chromosomes of the anaphase are changed into 

 the resting nucleus, and those by which they are condensed out of it 

 again in the following prophase. Moreover, a knowledge of these processes 

 is essential to a proper understanding of the relation between the structure 

 of the resting nucleus and that of the chromosomes. 



Unfortunately the telophase and early prophase are two of the most 

 difficult stages of the whole nuclear life-history to interpret, and very 

 different accounts of them have been given by different workers, some 

 of whom have claimed a large measure of generality for their conclusions, 

 explaining the conflicting results of other researches by faulty fixation, 

 wrong interpretation, etc. The contradictory conclusions reached by 

 different cytologists must indeed be attributed to these factors to a certain 

 extent, since they often refer to the same object. This is very well 

 illustrated by the work on three forms which have been much studied 

 on account of the size and clearness of their cytological elements, namely, 

 the tissue cells of the larval salamander, the developing eggs of Ascaris 

 megalocephala , and the root tips of the onion (Allium cepa). 



The egg of Ascaris is specially favourable for this study owing to its 

 small number of chromosomes, and to the fact that in the nuclear re- 

 construction at telophase the ends of the long chromosomes usually 

 form projections from the main mass of the nucleus, thus greatly facilitat- 

 ing the study of the changes taking place in a single chromosome (these 

 remarks refer to nuclei in the " germ track " only, see Chapter III.). 



Figs. 6, 7, 8 show the process by which the chromosomes pass into the 



