August 22, 1907] 



JVA TURE 



433 



sorting-out of the nuclear constituents originally furnished 

 by the two parents of the individual. This sorting or 

 distribution takes place in such a way that each of the 

 two daughter nuclei v\'hich arise as the result of the 

 division receives only half the total number of chromo- 

 somes previously conlribulcd by the two parents. The 

 essential point of interest lies in the fact that the process 

 does not consist in the mere halving of nuclear substance, 

 but in the distribution of nuclear constituents. When two 

 sexual cells which li^ive been formed in this way unite to 

 give rise to a new individual, the total number of nuclear 

 chromosomes is again made good ; but the resulting nuclear 

 constitution will not e.\actly resemble that of either parent. 

 That such is really the case is borne out by innumerable 

 experiments that have been made by breeders. Further- 

 more the extensive investigations on the results of crosses, 

 both in animals and plants, have confirmed the view that 

 particular characters can be treated as entities. For they 

 are distributed amongst the posterity of the original parents 

 in proportions that closely approximate to mathematical 

 expectation. In this distribution the separate characters 

 behave independently. For instance, the green colour and 

 round form of peas are two characters which may occur 

 in the same or in different individuals. The numerical 

 proportions in which they will appear can be foretold with 

 a considerable degree of accuracy. 



With these facts before us — and many others could be 

 adduced, all pointing in the same direction — it is not easy 

 to resist the conviction that within the nucleus there must 

 exist material entities which are severally responsible for 

 the appearance of the characteristic traits of any given 

 individual. The question is. What conception can we form 

 as to their nature, and how are they able to produce the 

 observed results? It is not necessary to discuss the 

 evidence that the chromosomes, or the materials of which 

 ihey are composed, play a most important part in connec- 

 tion with development. .All the work of the last decades 

 has tended to emphasise their importance in the trans- 

 mission of hereditary qualities, and this is equivalent to 

 admitting that they contain factors that determine the 

 path of development, and are responsible for the produc- 

 tion, from the egg, of the form and structure of the adult. 



Now it is certain that it is not the chromosome-substance 

 acting as a whole which is effective in those processes 

 summed up in the term Ontogeny. It might be, and until 

 recently was, thought that in those plants in which there 

 is a marked alternation of generations" a definite relation 

 existed between the number of the chromosomes and the 

 particular stage of the life-history. The double number 

 was supposed to be essential for the sporophyte, whilst 

 the halved number was similarly regarded as causally 

 related with the appearance of the gametophyte or pro- 

 thallial generation. 



But Loeb and others had already shown that the eggs 

 of echinoderms might be stimulated to parthcnogenetic 

 development by means other than fertilisation, and Wilson 

 found that such larv.t only contained the half number of 

 nuclear chromosomes, as, indeed, was only to be expected. 

 But 'the idea of a close parallelism between chromosome 

 number and the alternative phases of the life-history was 

 so deeply rooted that the full significance of Wilson's 

 discovery was not at once grasped. The comparative 

 neglect was, perhaps, partly justified, inasmuch as the 

 larvce could not be reared. It may, however, be incident- 

 ally remarked that no one, so far as I am aware, has yet 

 succeeded in raising the normal echinoderm larva beyond 

 the pluteus stage. 



The investigation of cases of apospory that occur in the 

 pteridophytes have proved that no causal relation can exist 

 between the number of the chromosomes and the characters 

 that distinguish the gametophyte and the sporophyte re- 

 spectively. For the sporophyte may give rise to the 

 gametophyte aposporously without any reduction, whilst 

 the various types of apogamy with which we are now 

 acquainted exhibit all gradations between a coalescence of 

 more or less differentiated nuclei and the complete absence 

 of all semblance of nuclear fusion. In the latter case, 

 when the sporophyte springs from a gametophyte that has 

 itself arisen after nuclear reduction, the sporophyte con- 

 tinues to retain the smaller number of chromosomes 

 normally associated with the other generation only. 



NO. 1973, "^'OL. ■ 6] 



We thus have a complete proof that a single sexual cell 

 which has ■ undergone reduction in the number of its 

 chromosomes retains, in so far as its architectural con- 

 figuration is concerned, the capacity of giving rise to a 

 plant possessed of the full complement of characters 

 belonging to the species. But this, after all, is only what 

 the facts of heredity miglit have led us to anticipate. For, 

 whilst we are ignorant of the fundamental significance of 

 the sexual fusion of the gametes, one of its most obvious 

 results consists in the duplication of the primordia of the 

 specific characters in the cells of the individual thus pro- 

 duced. This statement is not only in accord with results 

 of experiments in breeding, but it is also in harmonv with 

 the essential features of the heterotype mitosis ; and no 

 other satisfactory interpretation of the latter series of 

 phenomena has yet been found. 



Furthermore, the facts of Mendelian dominance clearly 

 show that each parent, through the gametes to which it 

 gives rise, contributes an independent organisation re- 

 sponsible for at least some of its own distinctive characters,, 

 as well as those which distinguish the species. Conse- 

 quently, when two gametes fuse, the embryo will be 

 provided with a duplicate stock of agents or primordia 

 which determine the appearance of its own specific and 

 individual characters. These will not always be similar 

 in the two parents, and when this is the case it often 

 happens that the offspring resembles one parent only in 

 respect of a particular feature. Nevertheless the results 

 of further breeding show that the corresponding, but 

 apparently lost, character only is latent, for it reappears 

 in a proportion — and often a fixed proportion — of the in- 

 dividuals of the succeeding generations. In such an 

 example, where both agents or primordia are present, one 

 of them lies dormant, whilst the dominant one alone 

 influences the course of metabolic processes, and thus 

 brings about the appearance of the character itself. The 

 dormant primordium can be transmitted as such through 

 many generations, betraying its existence in each by the 

 occurrence of individuals in which it finds its perfect 

 expression. This happens when the opposite dominant 

 agent or primordium has been removed from some of the 

 gametes by the sorting-out process during the heterotype 

 mitosis to which I have already alluded. 



I'he particulate character of inheritance seems, as man\' 

 writers have pointed out, to demand a structural organ- 

 isation for its basis ; and the units or primordia of which 

 the latter is composed must be relatively permanent. Inas- 

 much as heredity itself Is so stable. The agents or 

 primordia themselves probably act by definitely influencing 

 the course of chemical reactions that proceed within the 

 living protoplasm, somewhat after the fashion of the 

 ferments. But whether this influence on the course of 

 metabolism is to be attributed more directly to the 

 chemical or the physical aspect of the organisation must, 

 of course, remain an open question, though I incline to 

 the latter alternative on grounds which I have already 

 indicated. 



'l"he processes of the higher metabolism offer suggestive 

 analogies with those reactions for which the ferments are 

 responsible. In contemplating them one can hardly fall 

 to be struck by the orderly way in which ferment succeeds 

 ferment on an appiopriate medium. Each one produces its 

 own special change, which it is unable to carry further 

 itself, but it thereby provides a substratum suitable for its 

 successor. Starting, for example, with a complex sub- 

 stance like cane sugar, we see it acted on by a series of 

 ferments, each the result of protoplasmic differentiation, 

 and each one carrying the process of disintegration a little 

 further, but strictly limited In Its power to act, and only 

 able to take the change on to a definite stage. 



Everyone who lias experlmenled with plants with the 

 view of inducing the formation of some structure foreign 

 to the species or individual by artificial means must have 

 become impressed by the great difficulty of getting into 

 touch, so to speak, with the higher metabolism at all. 

 It is often easy enough to divert the life-history into 

 either the vegetative or the reproductive channel, as every 

 gardener is more or less consciously aware, and as Klebs 

 has conclusivelv shown in his remarkable series of care- 

 fully conducted experiments. But even here it is some- 

 times difficult exactly to hit off the conditions requisite 



