66 



NA rURE 



[May 19, 



would be really double. Be this as it may, these chromosomes 

 behave essentially like those of other preceding cells as regards 

 their fission, dividing longitudinally, as before. But the process 

 is here very complicated, and it is only as the result of very many 

 and careful researches that this fact has been definitely ascer- 

 tained. Quite apart from the altered (reduced) numbers of the 

 actual chromosomes present, the course of their development 

 deviates so widely from the normal type of karyokinesis in 

 whatever the animal or plant one may happen to be investigat- 

 ing, that it has been designated by Flemming as the Heterotype^ 

 division. 



It has already been stated that some writers hold that no true 

 reduction has occurred at this period, and by them (Hacker, 

 Rlickert, &c.), it is termed a pseudo-reduction, for they consider 

 that in the next, and rapidly following, division the real reduction 

 occurs. In the latter division it is believed, in the cases investi- 

 gated, which belong chiefly to the Arthropoda, that a real 

 qualitative reduction occurs by the splitting transversely of 

 each of the pseudo-chromosomes, and by the distribution of the 

 halves thus produced to the two daughter-nuclei. In other 

 words, the two genuine chromosomes which remain united as a 

 pseudo-chromosome during the heterotype karyokinesis, now 

 separate from each other, and thus each daughter-nucleus re- 

 ceives half the number of original entire chromosomes, and 

 consequently comes to contain slightly different sets of hereditary 

 potentialities. However this may be for Arthropoda, in which 

 the process is by no means easy to follow, it is certain, as the 

 researches of Meves, conducted under the auspices of Flemming, 

 clearly prove, that such a sorting of chromosomes does not occur 

 during the development of the sexual cells of Salamander, but 

 that the second (and last), like the heterotype division preceding 

 it, passes through a longitudinal-fission stage. And it is equally 

 certain that the same is true, at least, for the higher plants. 

 Ischikavva's recent results with AlHuni, which seem to point to 

 a contrary conclusion, can hardly be admitted as evidence one 

 way or another, since, judging from his own account of the pro- 

 cess, he seems to have misunderstood the stages with which he 

 was dealing. And in any case, the existence of numerous 

 exactly worked-out examples in which a transverse fission 

 certainly does not occur, obviously disposes of any attempt to 

 make it serve as the basis for a general theory of the mechanism 

 by which variation may be supposed to be secured. 



In spite of all the efforts which have been made, we are still 

 without a certain clue to the meaning of the reduction. Un- 

 questionably Weismann's view, which has been supported by 

 Hacker and others, oflfers the most attractive solution of the 

 puzzle ; but, as has been pointed out, it clearly will not ex- 

 plain the facts in all cases. Others believe the essential feature 

 to lie in the sudden reduction in the amount of chromatin 

 consequent on two so rapidly consecutive divisions But the 

 divisions do not invariably succeed each other with no interven- 

 ing period of rest. Strasburger has suggested that it represents 

 a return to an ancestral pre-fertilisation state, and it is possible 

 that there may be found to be some probability for this. But 

 against it is to be set the question why organisms with different 

 numbers of chromosomes in their nuclei always halve that 

 number, whatever it may happen to be, and do not all come to 

 possess a common number of reduced, and consequently of 

 duplicated chromosomes, for even closely related forms often 

 ■difler widely in this respect. However we explain it, it seems 

 clear that no theory which depends on the continued per- 

 manence of chromosomes can be admitted. Each one of the 

 reduced number cannot be compounded of two original ones, as 

 such, but must be a new structure ; else it is obvious that we have 

 no real reduction at all, but only a series of pseudo-reductions 

 — a view which would soon land us into an impossible position. 

 But if the chromosomes are not really permanent structures, 

 then the whole process of the two divisions of which we are 

 speaking, resolves itself into a mechanism which, whilst provid- 

 ing for a halving, provides equally for an accurate distribution 

 of the halved substance between the two final daughter-nuclei. 



1 The chief difFerences which distinguish the heterotype from other 

 divisions lies in (a) the long period of growth preparation ; (^) the rel.-uively 

 ■early appearance of longitudinal fission in the chromatic thread ; (y) the 

 frequent separation of the halves thus formed at this early stage, and their 

 subsequent approximation to one another of the halves in a variety of 

 ways, bejore they become grouped on the spindle ; (5) the curious and very 

 <:haracteristic appearance of the mature chromosohies on the spindle, some- 

 times taking that of closed rings, with (commonly) local equatorial thicken- 

 ing, or the cl romosonie at this stage may, in some forms, assume the 

 form of four spheres, loosely held together, constituting the vierer gt npfc 

 of the German authors. 



NO. 1490, VOL. 58] 



And although the acceptance of such a view of the matter would 

 involve a modification of those opinions shared by many as to 

 the nature of the architectural configuration of the hereditary 

 substance, in accordance with which discrete particles of it are 

 commonly assumed to be associated with definite hereditary 

 qualities, still the alternative hypothesis by no means negatives 

 the possibility of regarding heredity as the outcome of the con- 

 stitution of some such substance taken as a whole. The qualities 

 of the organism would depend on the structure of the material 

 basis, just as the structure of a crystal— to use an old illustration 

 — depends on the ultimate configuration of the constituent mole- 

 cules. An analogy of this sort is perhaps not worth much— it is 

 a comparison of a relatively simple with an infinitely complex 

 case — but still we are more likely to make a definite advance by 

 arguing, even imperfectly, from things of which we know some- 

 thing, than by abandoning ourselves to phantasies which are 

 intangible, and consequently incontestible. 



During the course of a nuclear division, there are few 

 phenomena which are more striking than the genesis and mode 

 of operation of that extraordinary structure known as the 

 achromatic spindle. This body provides the framework for the 

 whole process, as well as the machinery by which it is effected. 

 It originates in many different ways, and exhibits various degrees 

 of perfection in different organisms ; but the ultimate result 

 attained is much the same in all. 



Two extreme types of its modes of origin may be briefly out- 

 lined. In the less perfect form, as the periodof nuclear division 

 is about to commence, radiations are seen to start out in the 

 protoplasm. Sometimes these are connected with the nucleus 

 itself, but more often they seem to be f )cussed in groups on 

 many of the granules with which, at this stage, the protoplasin 

 is filled. But there is no sort of order in their arrangement. 

 Later on it is seen, however, that the lines become gradually 

 and with increasing rapidity focussed to two opposite spots in 

 the cell, and then the normal spindle is fully formed. It is idle 

 here to speak of the existence of centrosomes as initiating a 

 process which thus begins so irregularly, and the assumption 

 that they are really secretly existent all the time, and by their 

 hidden activity cause the astral radiations to converge to the 

 .two poles, suggests if not 2.petttio principii, at least a revelation 

 derived from some source from which mere mundane minds are 

 debarred. 



In the more perfectly formed mechanism, the spindle originates 

 from a definite mass of protoplasm which is intimately related 

 to centrosomes, and consequently it only is formed in this manner 

 when these structures are actually present. 



It appears to be, so to speak, spun out between the diverging 

 centrosomes, and either to pass into a groove in the nucleus, or 

 to rope up towards itself the chromosomes which by this lime 

 are differentiating. But whatever be the manner of its origin, 

 when it is fully formed it provides a structure upon which the 

 chromosomes are arrayed, and upon which, after the separation 

 of the two halves into which tht y severally split, the daughter 

 chromosomes travel to the respective ]5oles. In many cases 

 additional fibres can be distinguished which have become 

 attached to these retreating bodies, and thus, by contracting, 

 drag them towards the two ends of the spindle. 



The advantage, mechanically speaking, oT two poles to 

 which all the achromatic fibres running between them converge, 

 is clearly recognised during the changing conditions of stress 

 and strain which occur during the course of a karyokinesis, and 

 it serves to throw some light on certain phenomena which have 

 attracted less attention than they seem to deserve. 



Hitherto the poles have been treated of here as though 

 they were only marked by the convergence of the nuclear spindle 

 fibres ; but, over and above these, there are numerous other 

 fibres which radiate into the cell protoplasm, and which may 

 even reach the cell wall. Now, it is a significant fact that these 

 radiations are most apparent during the first formation of the 

 spindle and during the end phases of division, i.e. whilst the 

 daughter chromosomes are being pulled up to the poles. Often, 

 as in germinating spores of Pellia (a liverwort), they entirely 

 die away in the interval separating these two stages. The whole 

 appearance strongly suggests that the function of these radia- 

 tions, differentiated out of the cytoplasm, is to steady the poles, 

 and thus render the achromatic framework a rigid one. Indeed 

 without some such arrangement it is difficult, if one watches the 

 process going on, to imagine how the necessary stability would 

 be secured. 



When the chromosomes have reached their respective 



