darwin and modern science-第29章

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he　course　of　division。　　This　is　especially　striking　in　the　gonotokonts；　but　it　applies　also　to　the　somatic　tissues。　　In　the　latter；　one　usually　finds　twice　as　many　chromosomes　as　in　the　gonotokonts。　　Thus　the　conclusion　was　gradually　reached　that　the　doubling　of　chromosomes；　which　necessarily　accompanies　fertilisation；　is　maintained　in　the　product　of　fertilisation；　to　be　again　reduced　to　one　half　in　the　gonotokonts　at　the　stage　of　reduction…division。　　This　enabled　us　to　form　a　conception　as　to　the　essence　of　true　alternation　of　generations；　in　which　generations　containing　single　and　double　chromosomes　alternate　with　one　another。

The　single…chromosome　generation；　which　I　will　call　the　HAPLOID；　must　have　been　the　primitive　generation　in　all　organisms；　it　might　also　persist　as　the　only　generation。　　Every　sexual　differentiation　in　organisms；　which　occurred　in　the　course　of　phylogenetic　development；　was　followed　by　fertilisation　and　therefore　by　the　creation　of　a　diploid　or　double…　chromosome　product。　　So　long　as　the　germination　of　the　product　of　fertilisation；　the　zygote；　began　with　a　reducing　process；　a　special　DIPLOID　generation　was　not　represented。　　This；　however；　appeared　later　as　a　product　of　the　further　evolution　of　the　zygote；　and　the　reduction　division　was　correspondingly　postponed。　　In　animals；　as　in　plants；　the　diploid　generation　attained　the　higher　development　and　gradually　assumed　the　dominant　position。　　The　haploid　generation　suffered　a　proportional　reduction；　until　it　finally　ceased　to　have　an　independent　existence　and　became　restricted　to　the　role　of　producing　the　sexual　products　within　the　body　of　the　diploid　generation。　　Those　who　do　not　possess　the　necessary　special　knowledge　are　unable　to　realise　what　remains　of　the　first　haploid　generation　in　a　phanerogamic　plant　or　in　a　vertebrate　animal。　　In　Angiosperms　this　is　actually　represented　only　by　the　short　developmental　stages　which　extend　from　the　pollen　mother…cells　to　the　sperm…nucleus　of　the　pollen…tube；　and　from　the　embryo…sac　mother…cell　to　the　egg　and　the　endosperm　tissue。　　The　embryo…sac　remains　enclosed　in　the　diploid　ovule；　and　within　this　from　the　fertilised　egg　is　formed　the　embryo　which　introduces　the　new　diploid　generation。　　On　the　full　development　of　the　diploid　embryo　of　the　next　generation；　the　diploid　ovule　of　the　preceding　diploid　generation　is　separated　from　the　latter　as　a　ripe　seed。　　The　uninitiated　sees　in　the　more　highly　organised　plants　only　a　succession　of　diploid　generations。　　Similarly　all　the　higher　animals　appear　to　us　as　independent　organisms　with　diploid　nuclei　only。　　The　haploid　generation　is　confined　in　them　to　the　cells　produced　as　the　result　of　the　reduction　division　of　the　gonotokonts；　the　development　of　these　is　completed　with　the　homotypic　stage　of　division　which　succeeds　the　reduction　division　and　produces　the　sexual　products。

The　constancy　of　the　numbers　in　which　the　chromosomes　separate　themselves　from　the　nuclear　network　during　division　gave　rise　to　the　conception　that；　in　a　certain　degree；　chromosomes　possess　individuality。　　Indeed　the　most　careful　investigations　（Particularly　those　of　V。　Gregoire　and　his　pupils。）　have　shown　that　the　segments　of　the　nuclear　network；　which　separate　from　one　another　and　condense　so　as　to　produce　chromosomes　for　a　new　division；　correspond　to　the　segments　produced　from　the　chromosomes　of　the　preceding　division。　　The　behaviour　of　such　nuclei　as　possess　chromosomes　of　unequal　size　affords　confirmatory　evidence　of　the　permanence　of　individual　chromosomes　in　corresponding　sections　of　an　apparently　uniform　nuclear　network。　　Moreover　at　each　stage　in　division　chromosomes　with　the　same　differences　in　size　reappear。　　Other　cases　are　known　in　which　thicker　portions　occur　in　the　substance　of　the　resting　nucleus；　and　these　agree　in　number　with　the　chromosomes。　　In　this　network；　therefore；　the　individual　chromosomes　must　have　retained　their　original　position。　　But　the　chromosomes　cannot　be　regarded　as　the　ultimate　hereditary　units　in　the　nuclei；　as　their　number　is　too　small。　　Moreover；　related　species　not　infrequently　show　a　difference　in　the　number　of　their　chromosomes；　whereas　the　number　of　hereditary　units　must　approximately　agree。　　We　thus　picture　to　ourselves　the　carriers　of　hereditary　characters　as　enclosed　in　the　chromosomes；　the　transmitted　fixed　number　of　chromosomes　is　for　us　only　the　visible　expression　of　the　conception　that　the　number　of　hereditary　units　which　the　chromosomes　carry　must　be　also　constant。　　The　ultimate　hereditary　units　may；　like　the　chromosomes　themselves；　retain　a　definite　position　in　the　resting　nucleus。　　Further；　it　may　be　assumed　that　during　the　separation　of　the　chromosomes　from　one　another　and　during　their　assumption　of　the　rod…　like　form；　the　hereditary　units　become　aggregated　in　the　chromomeres　and　that　these　are　characterised　by　a　constant　order　of　succession。　　The　hereditary　units　then　grow；　divide　into　two　and　are　uniformly　distributed　by　the　fission　of　the　chromosomes　between　their　longitudinal　halves。

As　the　contraction　and　rod…like　separation　of　the　chromosomes　serve　to　isnure　the　transmission　of　all　hereditary　units　in　the　products　of　division　of　a　nucleus；　so；　on　the　other　hand；　the　reticular　distension　of　each　chromosome　in　the　so…called　resting　nucleus　may　effect　a　separation　of　the　carriers　of　hereditary　units　from　each　other　and　facilitate　the　specific　activity　of　each　of　them。

In　the　stages　preliminary　to　their　division；　the　chromosomes　become　denser　and　take　up　a　substance　which　increases　their　staining　capacity；　this　is　called　chromatin。　　This　substance　collects　in　the　chromomeres　and　may　form　the　nutritive　material　for　the　carriers　of　hereditary　units　which　we　now　believe　to　be　enclosed　in　them。　　The　chromatin　cannot　itself　be　the　hereditary　substance；　as　it　afterwards　leaves　the　chromosomes；　and　the　amount　of　it　is　subject　to　considerable　variation　in　the　nucleus；　according　to　its　stage　of　development。　　Conjointly　with　the　materials　which　take　part　in　the　formation　of　the　nuclear　spindle　and　other　processes　in　the　cell；　the　chromatin　accumulates　in　the　resting　nucleus　to　form　the　nucleoli。

Naturally　connected　with　the　conclusion　that　the　nuclei　are　the　carriers　of　hereditary　characters　in　the　organism；　is　the　question　whether　enucleate　organisms　can　also　exist。　　Phylogenetic　considerations　give　an　affirmative　answer　to　this　question。　　The　differentiation　into　nucleus　and　cytoplasm　represents　a　division　of　labour　in　the　protoplast。　　A　study　of　organisms　which　belong　to　the　lowest　class　of　the　organic　world　teaches　us　how　this　was　accomplished。　　Instead　of　well…defined　nuclei；　scattered　granules　have　been　described　in　the　protoplasm　of　several　of　these　organisms　（Bacteria；　Cyanophyceae；　Protozoa。）；　characterised　by　the　same　reactions　as　nuclear　material；　provided　also　with　a　nuclear　network；　but　without　a　limiting　membrane。　　（This　is　the　result　of　the　work　of　R。　Hertwig　and　of　the　most　recently　published　investigations。）　　Thus　the　carriers　of　hereditary　characters　may　originally　have　been　distributed　in　the　common　protoplasm；　afterwards　coming　together　and　eventually　assuming　a　definite　form　as　special　organs　of　the　cell。　　It　may　be　also　assumed　that　in　the　protoplasm　and　in　the　primitive　types　of　nucleus；　the　carriers　of　the　same　hereditary　unit　were　represented　in　considerable　quantity；　they　became　gradually　differentiated　to　an　extent　commensurate　with　newly　acquired　characters。　　It　was　also　necessary　that；　in　proportion　as　this　happened；　the　mechanism　of　nuclear　division　must　be　refined。　　At　first　processes　resembling　a　simple　constriction　would　suffice　to　provide　for　the　distribution　of　all　hereditary　units　to　each　of　the　products　of　division；　but　eventually　in　both　organic　kingdoms　nuclear　division；　which　alone　insured　the　qualitative　identity　of　the　products　of　division；　became　a　more　marked　feature　in　the　course　of　cell…multiplication。

Where　direct　nuclear　division　occurs　by　constriction　in　the　higher　organisms；　it　does　not　result　in　the　halving　of　hereditary　units。　　So　far　as　my　observations　go；　direct　nuclear　division　occurs　in　the　more　highly　organised　plants　only　in　cells　which　have　lost　their　specific　functions。　　Such　cells　are　no　longer　capable　of　specific　reproduction。　　An　interesting　case　in　this　connection　is　afforded　by　the　internodal　cells　of　the　Characeae；　which　possess　only　vegetative　functions。　　These　cells　grow　vigorously　and　their　cytoplasm　increases；　their　growth　being　accompanied　by　a　correspondingly　direct　multiplication　of　the　nuclei。　　They　serve　chiefly　to　nourish　the　plant；　but；　unlike　the　other　cells；　they　are　incapable　of　producing　any　offspring。　　This　is　a　very　instructive　case；　because　it　clearly　shows　that　the　nuclei　are　not　only　carriers　of　hereditary　characters；　but　that　they　also　play　a　definite　part　in　the　metabolism　of　the　protoplasts。

Attention　was　drawn　to　the　fact　that　during　the　reducing　division　of　nuclei　which　contain　chromosomes　of　unequal　size；　gemini　are　constantly　produced　by　the　pairing　of　chromosomes　of　the　same　size。　　This　led　to　the　conclusion　that　the　pairing　chromosomes　are　homologous；　and　that　one　comes　from　the　father；　the　other　from　the　mother。　　（First　stated　by　T。H。　Montgomery　in　1901　and　by　W。S。　Sutton　in　1902。）　　This　evidently　applies　also　to　the　pairing　of　chromosomes　in　those　reduction…divisions　in　which　differences　in　size　do　not　enable　us　to　distinguish　the　individual　chromosomes。　　In　this　case　also　each　pair　would　be　formed　by　two　homologous　chromosomes；　the　one　of　paternal；　the　other　of　maternal　origin。　　When　the　separation　of　these　chromosomes　and　their　distribution　to　both　daughter…nuclei　occur　a　chromosome　of　each　kind　is　provided　for　each　of　these　nuclei。　　It　would　seem　that　the　components　of　each　pair　might　pass　to　either　pole　of　the　nuclear　spindle；　so　that　the　paternal　and　maternal　chromosomes　would　be　distributed　in　varying　proportion　between　the　daughter…nuclei；　and　it　is　not　impossible　that　one　daughter…nucleus　might　occasionally　contain　paternal　chromosomes　only　and　its　sister…nucleus　exclusively　maternal　chromosomes。

The　fact　that　in　nuclei　containing　chromosomes　of　various　sizes；　the　chromosomes　which　pair　together　in　reduction…division　are　always　of　equal　size；　constitutes　a　further　and　more　important　proof　of　their　qualitative　difference。　　This　is　supported　also　by　ingenious　experiments　which　led　to　an　unequal　distribution　of　chromosomes　in　the　products　of　division　of　a　sea…urchin's　egg；　with　the　result　that　a　difference　was　induced　in　their　further　development。　　（Demonstrated　by　Th。　Boveri　in　1902。）

The　recently　discovered　fact　that　in　diploid　nuclei　the　chromosomes　are　arranged　in　pairs　affords　additional　evidence　in　favour　of　the　unequal　value　of　the　chromosomes。　　This　is　still　more　striking　in　the　case　of　chromosomes　of　different　sizes。　　It　has　been　shown　that　in　the　first　division…figure　in　the　nucleus　of　the　fertilised　egg　the　chromosomes　of　corresponding　size　form　pairs。　　They　appear　with　this　arrangement　in　all　subsequent　nuclear　divisions　in　the　diploid　generation。　　The　longitudinal　fissions　of　the　chromosomes　provide　for　the　unaltered　preservation　of　this　condition。　　In　the　reduction　nucleus　of　the　gonotokonts　the　homologous　chromosomes　being　near　together