1907.] Structural Constituents of the Nucleus, etc. 461 



beginning to prove untenable, to another that will perhaps turn out to be no 

 less vulnerable. But I think we are bound to take into account the structures 

 we can actually see, and the evolutions of which we can definitely follow, 

 before we take refuge in other and merely hypothetical entities. At any rate, 

 the plain facts of heredity, on the one hand, seem to postulate the existence 

 of large numbers of real units in the cell in order to account for the observed 

 phenomena, and, on the other hand, a study of the cell shows that such 

 structures are really present, although they may be crowded together into 

 larger or smaller distinct packets. And this will continue to be true, 

 whether the chromomeres, or still smaller particles, are adopted as the units. 



This view as to the variable nature of the chromosomes seems to me to 

 render intelligible certain other facts which have often proved puzzling, and 

 have become especially so during some recent investigations on the cytology 

 of ferns. In studying the nuclei of a series of varieties that have sprung, and 

 are still originating, from some of our common species, e.g., the Male Fern, 

 one cannot fail to be struck by the somewhat unexpected occurrence of 

 chromosome-numbers which show a great, but quite irregular, deviation from 

 that characteristic of the type. 



Sometimes the new numbers are in excess, sometimes they fall below that 

 which one would have anticipated. Thus in two varieties of the Male Fern 

 known as Polydactyla, the numbers in the prothallial cells are 66 and 90 

 respectively. The two ferns are remarkably alike in external features, as is 

 indicated by their common name ; but they are known to have originated as 

 distinct plants. In spite of their similarity, they show many points of 

 important difference when the various stages of their life-histories are 

 compared. They are both apogamous, and both replace the normal act 

 of fertilisation by the union of nuclei derived from adjacent vegetative cells, 

 in spite of the fact that they produce perfectly active sperms. One of them, 

 however, entirely lacks the female organ — the archegonium — whilst in the 

 other, though present, it is quite functionless. Along with these peculiar 

 features there is associated the change in the number of the chromosomes 

 already referred to. Now, it is important to notice that the differences do 

 not form multiples either of each other or of the number (72) of the typical 

 fern from which they are known to have originated. And when other 

 varieties are brought into the comparison, all grades of deviation are shown. 

 It would thus appear, since there is no corresponding diversity in the sizes 

 of the individual chromosomes, as though the variation must have arisen 

 through a re-arrangement of the whole substance of which they are made up ; 

 in other words, the chromomeres have become thrown into new combinations 

 quite apart from any further modifications they may have undergone. The 



2 L 2 



