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THE
AMER ICAN NATURALIST
VOL. XLVI (^) February, 1.912 No. 542
SOME ASPECTS OF^ CYTOLOGY^ IN^ RELATION
TO THE STUDY OF^ GENETICS'
PROFESSOR EDMUND B. WILSON COLUMBIA UNIVERSITY
THE considerationof geneticproblemsfromthestand-
point of cytological research sometimes encounters a certain opposition or prejudice which seems to me to be due to a misunderstandingof the position that is actu- ally held by many cytologists. It probably grows pri- marily out of a convictionthat the heredityof particular traits is not to be explained by referringthem to the operation of particular cell-elementsor "determiners," but results from^ an activity of the whole cell-system,or of the whole organism, regarded as a unit. With this view, as will appear, I am essentially in agreement. In the second place, the opposition is a kind of reaction or protest against the theory of pangens or biophores and the too elaborate logical constructionsthat have been built upon it, especially by Weismann. I also consider this theory untenable, or at least unnecessary. I will thereforeattemptto outline a point of view fromwhich I think genetic problems may reasonably be regarded from the standpoint of the cytologist. The most essential result of modern genetic inquiry I take to be the proof of the independence of the so-called (^1) A paper read before the American' Society of Naturalists at the Princeton meeting, December 28, 1911. 57
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"unit-characters' (^) that is to say, that theymay be inde- pendently combined, disassociated and re-combined in many differentways. The independence of these char- acters often seems to be complete; more rarely it is limited by definitephenomena of "coupling" or "repul- sion." The interestingfacts recentlybroughtto light by Bateson and Punnett in case of certain unit-characters in plants, and by Morgan in case of sex-limited char- acters in flies, demonstrate that coupling or repulsion, as exhibited in the F2 (^) generation, are a consequence of an original association or separationin the,grandparental gametes. In the cases referredto, characters that enter the F1 zygote in the same gamete tend to "couple" (re- main in association) in the gametes produced by this gen- eration; while if these same characters enter the F. zygote in differentgametes they tend to "repel" each other (remain separate) in the ensuing gamete-forma- tion. This is almost a proof that the factors for coupled characters are borne by a commonvehicle or substratum in the germ-cell,while in repulsion they are borne by separate ones. Not alone such facts, lut the whole history of unit-characterspoints unmistakably to the conclusion that thev are in some way connected witli material substances or bodies; and that it is the com- binations,disassociations and recombinationsof the latter that explain the correspondingbehavior of the former. For example, in sex-limitedhereditythe peculiar linkage of certain unit-characterswith sex becomes readily intel- ligible, as several writers have recentlypointed out, if factors necessary for the production of these characters are associated in the same material body with a factor that plays a certain necessary role in the production of sex. In this particular case, as it happens, we are actu- ally able to see a material body (the "X-chromosome") which undergoes precisely such a mode of distribution with respect to sex and sex-limitedcharacters as is de- manded by the hypothesis. The question must here be squarely faced, in a very real and concreteform,whether
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kind of chemical entitywhich induces a specificreaction of the germ sooner or later in its development. But beyondthis it is perfectlyevidentthat however^ far back- wards we may follow such a series of unit-factors,at every stage theyplay their specificrole only in so far as they form part of a still more general apparatus of ontogeneticreaction that is constitutedby the organism as a whole. The whole of this apparatus, the entire
germinii-?alcomtplex, is directly or inzdirectly involved iFe
theproductionof everycharacter. We findit convenient, indeed necessary, to treat particular factors of reaction (i. e., the "determiners") as if they were concrete and separate things. Such, iti fact, they mayTbe, as already indicated; but when -wespeak of them as "bearers" of the corresponding characters, we are -usinga figure of speech that may be highly misleading. The reactions (characters) which they call forth are not "borne by" them. They appear as responses of the germinal organi- zation operating as a unit-system;and it is to this sys- tem as a whole that every character belongs, or by which it is (^) "borne"-if indeed we may permit ourselves to employthe latter expression at all. The point of view thus indicated may, I think,be made entirely clear by a chemical illustration. A number of writers, among them Adami, Guyer and Kossel, have of late called attentionto the parallel that may be drawn between the physical basis of heredityand the complex molecular groups of the proteins and other (^) organic com- pounds. It is a most suggestive one, though it is not to be taken too literally-indeed I shall employ it only as a kind of allegory or illustrative fiction. No one can fail to be struck with the really remarkable analogy, in method and in results, between the procedure of modern genetic experiment and that of modern organic chem- istry. Just as the qualities of a particular protein may be definitelyaltered by the addition, subtraction or the substitutionone for another of particular side-chains or molecular "Bausteine," so the addition, subtraction or
No. 542] SOMHEASPECTS OF CYTOLOGY 61
substitutionof particular " determiners" or "factors" in the zygote calls forth specificresponses that lead to the production of correspondingcharacters. The reason- ing that applies to the firstof these cases seems equally applicable to the second. No one, I suppose, would hold in the firstcase that the particular molecular groups or "IBausteine" concernedin the change are "bearers of" (i. e., are alone responsible for) the resultingnew quali- ties. The qualities of any protein,as Kossel has recently urged, belong to the molecule as a whole, and are not to be regarded as the sum of the qualities of its constituent "Bausteine. " Why should we regard in a differentlight the "determiners" (chemical substances?) concerned in the second case? They are (^) clearly, not to be regarded as "bearers" or ''physical bases" of the characters which depend upon their presence or absence. They are, I repeat, only differentialfactors of ontogenetic reac- tions that belong to the germ considered as a whole or unit-system. In all this I am but expressing what I believe to be the point of view of many recent writers on genetic prob- lens; but what I^ desire to emphasize is that the prob- leml-sof cytologyshould be regarded fromthe same point of view. It is our task to see whether an apparatus of ontogen etic response can be discovered in the cell that fitswith such a conception of the general process of de- termination. Is there cytological evidence of the exist- ence in the germ-cellof such specificfactors of reaction as I have referredto-in the nucleus, in the protoplasm, or in both? I think that observation and experiment alike have (^) prodUced such. evidence. Such (^) experiments as those of Boveri on multipolarmitosis,and of Herbst and of Baltzer on the relations of the chromosomesin recip- rocal crosses in sea-urchins have almost conclusively shown that the chromatindoes in fact play a causative role in determination. Observation has gradually estab- lished the existence of a complex process of segregation and distributionof the nuclear materials in lkaryokinesis,
No. 542] SOME ASPECTS OF CYTOLOGY 63
that the spireme-threadsoftenconsist of linear series of ,granules. I have iii hind the fact that the number and size-relationsof the chromosomesoftendiffermaterially as betweendifferentspecies, evennearlyrelated ones, and that in at least one case (that of the X-chromosome)it is an established fact that a particular chromosomewhich in some species is a single body may be represented^ in other species by two or more componentsthat sometimes show constantand characteristicdifferencesof size.^ The natural interpretationof this fact is that the ehromo- somes are compound bodies, consisting of differentcon- stituentswhich undergo differentmodes of segregation in differentspecies. We may here finda rational expla- nation, both of sex-limitedheredity,as I have elsewhere indicated, and of otherkinds of coupling.
- Recent studies on lkarvokinesisand maturation em- phasize anew the importailce of the mitotictransforma- tion of the chroniatin-substance,and acid weight to Roux 's original interpretation of this (^) phelonoenon. Nothing in recent cytological research is more interest-
ing thanthe discoveryby Bonnevie,Pinney,DavTis, and
others that new chromosomesmay arise within the old ones in the form of tightlycoiled or convolutedthreads, wwhichuncoil or unravel to formnseparate spiremle-threads.
In karyokineticdivision these threads may be formed
already inside the telophase-chromnosomesof the preced- ing division, as was discovered by Bonnevie. In other cases, an example of^ whichis givenby certain Orthoptera firstreportedupon by Miss Pinney,^ they^ armfirstvisible in the early prophases, when they are seen uncoiling from massive bodies formed^ from^ the old chromosomes and equal in nummberto them. The same remarkable process occurs in the^ early auxocytes,as the^ chromosomes are preparing for conjugation in synapsis, as has been shown particularly by Davis,^ whose observations, like those of Pinney, I^ have recently been able to confirm and extend. In many insects the presynaptic spireme- threads do not arise, as has oftenbeen described^ in other
64 THE AMHERICAN NATURALIST [VOL. XLVI
forms, directly from a chromatin-network.They arise frommassive bodies, each of which resolves itself into a closely convolutedthread which then uncoils before con- jugation takes place. Why should chromosomesthat are already formed as massive bodies delay their division or conjugation until so remarkable a redistributionof their substance has taken place? It is not a necessary conditionof conjugation,as is proved by the case of both the sex-chromosomesand the m-chromi-osomes,which do in fact conjugate in the massive condition. All the facts become intelligiblein the light of Roux's hypothesisthat the formation of the spireme-threads effects a linear alignmentof differentconstituentsin preparation either for division or for a definitetype of association in pairs in synapses. One of the most interestingapplications of this view to genetic phenomena is that suggested by Jansseiis in his theory of the "chiasniatype," which has recently been applied by Morgan to the explanation of coupling and repulsion. In the twisting together of the spireme threads, either in synapsis or at a succeeding stage, and the subsequent secondary splitting of the thread in one plane is provided a very simple mechanical basis, both for the free interchange of factors between the homol- ogous chromosomesand for the phenomena of coupling or repulsion, which are otherwise so difficultto compre- hend. I do not maintain that this particular interpreta- tion, or the more general one of Roux, is demonstrably true, or that no other explanation can be found. I only hold that they are legitimate conceptions which may be tested by observationand experiment,and whichmust be fully reckoned with as intelligibleinterpretationsof the facts before we can set these facts aside as utterlymys- terious or as a meaningless by-play.
- I would lastly recall the experimental proof by Boveri that the chromosomesdifferamong themselvesin their (^) physiological relation to development,and the cor- respondingcytologicalfact that they differamong them-
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nuclear organization as have here been indicated are both vague and artificial. Vague and crude theyundoubtedly are; (^) and so they will remain until we (^) have far more thoroughlyexplored a fieldof inquiry in which we must for the present make a shiftwith crude ideas unless we are contentto work with no ideas at all. They may be artificial too; but it appears to me that in this respect they differonly in degree from the graphic formulas of structural chemlistry. The chemist does not hesitate to picture definitetopographlicalor spatial relations in the complexorgganicmolecule symmetricaland asymmetrical foris, cyclic or ring-formations,linear^ series, side- chains and other such graphic constructions. It is by their use that the whole science of organic chemistryhas been 1)Uiltup, and that such men as Emil Fischer and Kossel have made nearly all of their advances in our knowledgeof those most complex of known organic comn- pounds the proteins. And these constructionsare re- garcdedby eminentinvestigatorsas somethingmore than mere figurativeexpressions or symbols. They are taken more literally as representations or models rude, no doubt, but as far as they go real of the actual arrange- ments in space of the various molecular groups or pro- tein "B austeine." If thereforeobservation and experi- ment lead the cytologistto postulate definitetopograph- ical relations among the nuclear substances, and if such a conception help him to explain the results of genetic studies, lie findshimself in good company, even though his present clumsynotions regarding the nuclear organi- zation can as yet make no approach to the exact and ele- gant constructionsof^ the chemist. The essential conclusion that is indicated by cytolog- ical study of the nuclear substance^ is that it is an aggre- gate of many differentchemical components,which do not constitutea mere mechanical^ mixture,but a complex organic system, and which undergo perfectly ordered processes of segregation and distributionin the cycle of cell-life. That these substances play some definiterole
No. 542] SOME ASPECTS' OF CYTOLOGY 67
in determinationis not a mere assumption,but a conclu- sion based upon direct cytological experiment,and one that finds support in the results of modern chemical research. Professor Kossel has recentlysaid that every peculiarity of the species and every occurrenceaffecting the individual may be indicated by special combinations of protein "'B austeine.'' The point of vTiew that has here been indicated is entirelyin accordance with such a con- ception. The results of cytological inquiry fit with the vTiew that there are many such combinations in both nucleus and protoplasm; and the interest of cyTtological study.lies in the fact that we can in some degree follow out their modes of segregation and distributionwith the microscope. We are still utterlyignorantas to how these processes are determined;and the more one studies them the more one's wonder grows. I would certainlyTbe one of the last to disparage the brilliant results that have been attained throughthe prolonged and patient labors of cytologicalobservation and experiment. They stand, I believe, among the most interesting and vTaluable achievementsof niodernbiologyc. But these studies have as vet made no approach to their limit,and a vast unex- plored fieldstill lies beforeus. We may as well recognize the fact that our present rude notions of cell-organiza- tion have not yet progressed very far beyond the paleo- lithic stage of culture; but they are of use in so far as they help to open new points of view or to discover new facts,whetherin cytologicor in geneticinquiry. It seems to me that in both regards they have already proved worth while.
