darwin and modern science-第39章

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ses；　and　were　distinguished　from　the　direct　or　amitotic　divisions　which　are　characterised　by　a　simple　constriction　of　the　nuclear　body。　　So　long　as　the　two　kinds　of　indirect　nuclear　division　were　not　clearly　distinguished；　their　correct　interpretation　was　impossible。　　This　was　accomplished　after　long　and　laborious　research；　which　has　recently　been　carried　out　and　with　results　which　should；　perhaps；　be　regarded　as　provisional。

Soon　after　the　new　study　of　the　nucleus　began；　investigators　were　struck　by　the　fact　that　the　course　of　nuclear　division　in　the　mother…cells；　or　more　correctly　in　the　grandmother…cells；　of　spores；　pollen…grains；　and　embryo…　sacs　of　the　more　highly　organised　plants　and　in　the　spermatozoids　and　eggs　of　the　higher　animals；　exhibits　similar　phenomena；　distinct　from　those　which　occur　in　the　somatic　cells。

In　the　nuclei　of　all　those　cells　which　we　may　group　together　as　gonotokonts　（At　the　suggestion　of　J。P。　Lotsy　in　1904。）　（i。e。　cells　concerned　in　reproduction）　there　are　fewer　chromosomes　than　in　the　adjacent　body…cells　（somatic　cells）。　　It　was　noticed　also　that　there　is　a　peculiarity　characteristic　of　the　gonotokonts；　namely　the　occurrence　of　two　nuclear　divisions　rapidly　succeeding　one　another。　　It　was　afterwards　recognised　that　in　the　first　stage　of　nuclear　division　in　the　gonotokonts　the　chromosomes　unite　in　pairs：　　it　is　these　chromosome…pairs；　and　not　the　two　longitudinal　halves　of　single　chromosomes；　which　form　the　nuclear　plate　in　the　equatorial　plane　of　the　nuclear　spindle。　　It　has　been　proposed　to　call　these　pairs　gemini。　　（J。E。S。　Moore　and　A。L。　Embleton；　〃Proc。　Roy。　Soc。〃　London；　Vol。　LXXVII。　page　555；　1906；　V。　Gregoire；　1907。）　　In　the　course　of　this　division　the　spindle…fibrillae　attach　themselves　to　the　gemini；　i。e。　to　entire　chromosomes　and　direct　them　to　the　points　where　the　new　daughter…　nuclei　are　formed；　that　is　to　those　positions　towards　which　the　longitudinal　halves　of　the　chromosomes　travel　in　ordinary　nuclear　divisions。　　It　is　clear　that　in　this　way　the　number　of　chromosomes　which　the　daughter…nuclei　contain；　as　the　result　of　the　first　stage　in　division　in　the　gonotokonts；　will　be　reduced　by　one　half；　while　in　ordinary　divisions　the　number　of　chromosomes　always　remains　the　same。　　The　first　stage　in　the　division　of　the　nucleus　in　the　gonotokonts　has　therefore　been　termed　the　reduction　division。　　（In　1887　W。　Flemming　termed　this　the　heterotypic　form　of　nuclear　division。）　　This　stage　in　division　determines　the　conditions　for　the　second　division　which　rapidly　ensues。　　Each　of　the　paired　chromosomes　of　the　mother…nucleus　has　already；　as　in　an　ordinary　nuclear　division；　completed　the　longitudinal　fission；　but　in　this　case　it　is　not　succeeded　by　the　immediate　separation　of　the　longitudinal　halves　and　their　allotment　to　different　nuclei。　　Each　chromosome；　therefore；　takes　its　two　longitudinal　halves　into　the　same　daughter…nucleus。　　Thus；　in　each　daughter…nucleus　the　longitudinal　halves　of　the　chromosomes　are　present　ready　for　the　next　stage　in　the　division；　they　only　require　to　be　arranged　in　the　nuclear　plate　and　then　distributed　among　the　granddaughter…nuclei。　This　method　of　division；　which　takes　place　with　chromosomes　already　split；　and　which　have　only　to　provide　for　the　distribution　of　their　longitudinal　halves　to　the　next　nuclear　generation；　has　been　called　homotypic　nuclear　division。　　（The　name　was　proposed　by　W。　Flemming　in　1887；　the　nature　of　this　type　of　division　was；　however；　not　explained　until　later。）

Reduction　division　and　homotypic　nuclear　division　are　included　together　under　the　term　allotypic　nuclear　division　and　are　distinguished　from　the　ordinary　or　typical　nuclear　division。　　The　name　Meiosis　（By　J。　Bretland　Farmer　and　J。E。S。　Moore　in　1905。）　has　also　been　proposed　for　these　two　allotypic　nuclear　divisions。　　The　typical　divisions　are　often　spoken　of　as　somatic。

Observers　who　were　actively　engaged　in　this　branch　of　recent　histological　research　soon　noticed　that　the　chromosomes　of　a　given　organism　are　differentiated　in　definite　numbers　from　the　nuclear　network　in　the　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　o