darwin and modern science-第34节

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ytology　and　cannot　be　at　variance　with　cytological　facts。　　Many　histological　discoveries；　both　such　as　have　been　proved　correct　and　others　which　may　be　accepted　as　probably　well　founded；　have　acquired　a　fundamental　importance　from　the　point　of　view　of　the　problems　of　heredity。

My　aim　is　to　describe　the　present　position　of　our　knowledge　of　Cytology。　　The　account　must　be　confined　to　essentials　and　cannot　deal　with　far…　reaching　and　controversial　questions。　　In　cases　where　difference　of　opinion　exists；　I　adopt　my　own　view　for　which　I　hold　myself　responsible。　　I　hope　to　succeed　in　making　myself　intelligible　even　without　the　aid　of　illustrations：　　in　order　to　convey　to　the　uninitiated　an　adequate　idea　of　the　phenomena　connected　with　the　life　of　a　cell；　a　greater　number　of　figures　would　be　required　than　could　be　included　within　the　scope　of　this　article。

So　long　as　the　most　eminent　investigators　（As　for　example　the　illustrious　Wilhelm　Hofmeister　in　his　〃Lehre　von　der　Pflanzenzelle〃　（1867）。）　believed　that　the　nucleus　of　a　cell　was　destroyed　in　the　course　of　each　division　and　that　the　nuclei　of　the　daughter…cells　were　produced　de　novo；　theories　of　heredity　were　able　to　dispense　with　the　nucleus。　　If　they　sought；　as　did　Charles　Darwin；　who　showed　a　correct　grasp　of　the　problem　in　the　enunciation　of　his　Pangenesis　hypothesis；　for　histological　connecting　links；　their　hypotheses；　or　at　least　the　best　of　them；　had　reference　to　the　cell　as　a　whole。　　It　was　known　to　Darwin　that　the　cell　multiplied　by　division　and　was　derived　from　a　similar　pre…existing　cell。　　Towards　1870　it　was　first　demonstrated　that　cell…nuclei　do　not　arise　de　novo；　but　are　invariably　the　result　of　division　of　pre…existing　nuclei。　　Better　methods　of　investigation　rendered　possible　a　deeper　insight　into　the　phenomena　accompanying　cell　and　nuclear　divisions　and　at　the　same　time　disclosed　the　existence　of　remarkable　structures。　　The　work　of　O。　Butschli；　O。　Hertwig；　W。　Flemming　H。　Fol　and　of　the　author　of　this　article　（For　further　reference　to　literature；　see　my　article　on　〃Die　Ontogenie　der　Zelle　seit　1875〃；　in　the　〃Progressus　Rei　Botanicae〃；　Vol。　I。　page　1；　Jena；　1907。）；　have　furnished　conclusive　evidence　in　favour　of　these　facts。　　It　was　found　that　when　the　reticular　framework　of　a　nucleus　prepares　to　divide；　it　separates　into　single　segments。　　These　then　become　thicker　and　denser；　taking　up　with　avidity　certain　stains；　which　are　used　as　aids　to　investigation；　and　finally　form　longer　or　shorter；　variously　bent；　rodlets　of　uniform　thickness。　　In　these　organs　which；　on　account　of　their　special　property　of　absorbing　certain　stains；　were　styled　Chromosomes　（By　W。　Waldeyer　in　1888。）；　there　may　usually　be　recognised　a　separation　into　thicker　and　thinner　discs；　the　former　are　often　termed　Chromomeres。　　（Discovered　by　W。　Pfitzner　in　1880。）　　In　the　course　of　division　of　the　nucleus；　the　single　rows　of　chromomeres　in　the　chromosomes　are　doubled　and　this　produces　a　band…like　flattening　and　leads　to　the　longitudinal　splitting　by　which　each　chromosome　is　divided　into　two　exactly　equal　halves。　　The　nuclear　membrane　then　disappears　and　fibrillar　cell…plasma　or　cytoplasm　invades　the　nuclear　area。　　In　animal　cells　these　fibrillae　in　the　cytoplasm　centre　on　definite　bodies　（Their　existence　and　their　multiplication　by　fission　were　demonstrated　by　E。　van　Beneden　and　Th。　Boveri　in　1887。）；　which　it　is　customary　to　speak　of　as　Centrosomes。　　Radiating　lines　in　the　adjacent　cell…plasma　suggest　that　these　bodies　constitute　centres　of　force。　　The　cells　of　the　higher　plants　do　not　possess　such　individualised　centres；　they　have　probably　disappeared　in　the　course　of　phylogenetic　development：　　in　spite　of　this；　however；　in　the　nuclear　division…figures　the　fibrillae　of　the　cell…plasma　are　seen　to　radiate　from　two　opposite　poles。　　In　both　animal　and　plant　cells　a　fibrillar　bipolar　spindle　is　formed；　the　fibrillae　of　which　grasp　the　longitudinally　divided　chromosomes　from　two　opposite　sides　and　arrange　them　on　the　equatorial　plane　of　the　spindle　as　the　so…　called　nuclear　or　equatorial　plate。　　Each　half…chromosome　is　connected　with　one　of　the　spindle　poles　only　and　is　then　drawn　towards　that　pole。　　（These　important　facts；　suspected　by　W。　Flemming　in　1882；　were　demonstrated　by　E。　Heuser；　L。　Guignard；　E。　van　Beneden；　M。　Nussbaum；　and　C。　Rabl。）

The　formation　of　the　daughter…nuclei　is　then　effected。　　The　changes　which　the　daughter…chromosomes　undergo　in　the　process　of　producing　the　daughter…　nuclei　repeat　in　the　reverse　order　the　changes　which　they　went　through　in　the　course　of　their　progressive　differentiation　from　the　mother…nucleus。　　The　division　of　the　cell…body　is　completed　midway　between　the　two　daughter…　nuclei。　　In　animal　cells；　which　possess　no　chemically　differentiated　membrane；　separation　is　effected　by　simple　constriction；　while　in　the　case　of　plant　cells　provided　with　a　definite　wall；　the　process　begins　with　the　formation　of　a　cytoplasmic　separating　layer。

The　phenomena　observed　in　the　course　of　the　division　of　the　nucleus　show　beyond　doubt　that　an　exact　halving　of　its　substance　is　of　the　greatest　importance。　　（First　shown　by　W。　Roux　in　1883。）　　Compared　with　the　method　of　division　of　the　nucleus；　that　of　the　cytoplasm　appears　to　be　very　simple。　　This　led　to　the　conception　that　the　cell…nucleus　must　be　the　chief　if　not　the　sole　carrier　of　hereditary　characters　in　the　organism。　　It　is　for　this　reason　that　the　detailed　investigation　of　fertilisation　phenomena　immediately　followed　researches　into　the　nucleus。　　The　fundamental　discovery　of　the　union　of　two　nuclei　in　the　sexual　act　was　then　made　（By　O。　Hertwig　in　1875。）　and　this　afforded　a　new　support　for　the　correct　conception　of　the　nuclear　functions。　　The　minute　study　of　the　behaviour　of　the　other　constituents　of　sexual　cells　during　fertilisation　led　to　the　result；　that　the　nucleus　alone　is　concerned　with　handing　on　hereditary　characters　（This　was　done　by　O。　Hertwig　and　the　author　of　this　essay　simultaneously　in　1884。）　from　one　generation　to　another。　　Especially　important；　from　the　point　of　view　of　this　conclusion；　is　the　study　of　fertilisation　in　Angiosperms　（Flowering　plants）；　in　these　plants　the　male　sexual　cells　lose　their　cell…body　in　the　pollen…tube　and　the　nucleus　only　the　sperm…nucleusreaches　the　egg。　　The　cytoplasm　of　the　male　sexual　cell　is　therefore　not　necessary　to　ensure　a　transference　of　hereditary　characters　from　parents　to　offspring。　　I　lay　stress　on　the　case　of　the　Angiosperms　because　researches　recently　repeated　with　the　help　of　the　latest　methods　failed　to　obtain　different　results。　　As　regards　the　descendants　of　angiospermous　plants；　the　same　laws　of　heredity　hold　good　as　for　other　sexually　differentiated　organisms；　we　may；　therefore；　extend　to　the　latter　what　the　Angiosperms　so　clearly　teach　us。

The　next　advance　in　the　hitherto　rapid　progress　in　our　knowledge　of　nuclear　division　was　delayed；　because　it　was　not　at　once　recognised　that　there　are　two　absolutely　different　methods　of　nuclear　division。　　All　such　nuclear　divisions　were　united　under　the　head　of　indirect　or　mitotic　divisions；　these　were　also　spoken　of　as　karyo…kineses；　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　gonotok