darwin and modern science-第65章

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ery　efficient　in　this　direction。　　It　was　also　found　that　the　higher　acids　are　more　efficient　than　the　lower　ones；　and　it　is　possible　that　the　spermatozoon　induces　membrane…formation　by　carrying　into　the　egg　a　higher　fatty　acid；　namely　oleic　acid　or　one　of　its　salts　or　esters。

The　physico…chemical　process　which　underlies　the　formation　of　the　membrane　seems　to　be　the　cause　of　the　development　of　the　egg。　　In　all　cases　in　which　the　unfertilised　egg　has　been　treated　in　such　a　way　as　to　cause　it　to　form　a　membrane　it　begins　to　develop。　　For　the　eggs　of　certain　animals　membrane…　formation　is　all　that　is　required　to　induce　a　complete　development　of　the　unfertilised　egg；　e。g。　in　the　starfish　and　certain　annelids。　　For　the　eggs　of　other　animals　a　second　treatment　is　necessary；　presumably　to　overcome　some　of　the　injurious　effects　of　acid　treatment。　　Thus　the　unfertilised　eggs　of　the　sea…urchin　Strongylocentrotus　purpuratus　of　the　Californian　coast　begin　to　develop　when　membrane…formation　has　been　induced　by　treatment　with　a　fatty　acid；　e。g。　butyric　acid；　but　the　development　soon　ceases　and　the　eggs　perish　in　the　early　stages　of　segmentation；　or　after　the　first　nuclear　division。　　But　if　we　treat　the　same　eggs；　after　membrane…　formation；　for　from　35　to　55　minutes　（at　15　deg　C。）　with　sea…water　the　concentration　（osmotic　pressure）　of　which　has　been　raised　through　the　addition　of　a　definite　amount　of　some　salt　or　sugar；　the　eggs　will　segment　and　develop　normally；　when　transferred　back　to　normal　sea…water。　　If　care　is　taken；　practically　all　the　eggs　can　be　caused　to　develop　into　plutei；　the　majority　of　which　may　be　perfectly　normal　and　may　live　as　long　as　larvae　produced　from　eggs　fertilised　with　sperm。

It　is　obvious　that　the　sea…urchin　egg　is　injured　in　the　process　of　membrane…formation　and　that　the　subsequent　treatment　with　a　hypertonic　solution　only　acts　as　a　remedy。　　The　nature　of　this　injury　became　clear　when　it　was　discovered　that　all　the　agencies　which　cause　haemolysis；　i。e。　the　destruction　of　the　red　blood　corpuscles；　also　cause　membrane…formation　in　unfertilised　eggs；　e。g。　fatty　acids　or　ether；　alcohols　or　chloroform；　etc。；　or　saponin；　solanin；　digitalin；　bile　salts　and　alkali。　　It　thus　happens　that　the　phenomena　of　artificial　parthenogenesis　are　linked　together　with　the　phenomena　of　haemolysis　which　at　present　play　so　important　a　role　in　the　study　of　immunity。　　The　difference　between　cytolysis　（or　haemolysis）　and　fertilisation　seems　to　be　this；　that　the　latter　is　caused　by　a　superficial　or　slight　cytolysis　of　the　egg；　while　if　the　cytolytic　agencies　have　time　to　act　on　the　whole　egg　the　latter　is　completely　destroyed。　　If　we　put　unfertilised　eggs　of　a　sea…urchin　into　sea…water　which　contains　a　trace　of　saponin　we　notice　that；　after　a　few　minutes；　all　the　eggs　form　the　typical　membrane　of　fertilisation。　　If　the　eggs　are　then　taken　out　of　the　saponin　solution；　freed　from　all　traces　of　saponin　by　repeated　washing　in　normal　sea…water；　and　transferred　to　the　hypertonic　sea…water　for　from　35　to　55　minutes；　they　develop　into　larvae。　　If；　however；　they　are　left　in　the　sea…water　containing　the　saponin　they　undergo；　a　few　minutes　after　membrane…formation；　the　disintegration　known　in　pathology　as　CYTOLYSIS。　　Membrane…formation　is；　therefore；　caused　by　a　superficial　or　incomplete　cytolysis。　　The　writer　believes　that　the　subsequent　treatment　of　the　egg　with　hypertonic　sea…water　is　needed　only　to　overcome　the　destructive　effects　of　this　partial　cytolysis。　　The　full　reasons　for　this　belief　cannot　be　given　in　a　short　essay。

Many　pathologists　assume　that　haemolysis　or　cytolysis　is　due　to　a　liquefaction　of　certain　fatty　or　fat…like　compounds；　the　so…called　lipoids；　in　the　cell。　　If　this　view　is　correct；　it　would　be　necessary　to　ascribe　the　fertilisation　of　the　egg　to　the　same　process。

The　analogy　between　haemolysis　and　fertilisation　throws；　possibly；　some　light　on　a　curious　observation。　　It　is　well　known　that　the　blood　corpuscles；　as　a　rule；　undergo　cytolysis　if　injected　into　the　blood　of　an　animal　which　belongs　to　a　different　family。　　The　writer　found　last　year　that　the　blood　of　mammals；　e。g。　the　rabbit；　pig；　and　cattle；　causes　the　egg　of　Strongylocentrotus　to　form　a　typical　fertilisation…membrane。　　If　such　eggs　are　afterwards　treated　for　a　short　period　with　hypertonic　sea…water　they　develop　into　normal　larvae　（plutei）。　　Some　substance　contained　in　the　blood　causes；　presumably；　a　superficial　cytolysis　of　the　egg　and　thus　starts　its　development。

We　can　also　cause　the　development　of　the　sea…urchin　egg　without　membrane…　formation。　　The　early　experiments　of　the　writer　were　done　in　this　way　and　many　experimenters　still　use　such　methods。　　It　is　probable　that　in　this　case　the　mechanism　of　fertilisation　is　essentially　the　same　as　in　the　case　where　the　membrane…formation　is　brought　about；　with　this　difference　only；　that　the　cytolytic　effect　is　less　when　no　fertilisation…membrane　is　formed。　This　inference　is　corroborated　by　observations　on　the　fertilisation　of　the　sea…urchin　egg　with　ox　blood。　　It　very　frequently　happens　that　not　all　of　the　eggs　form　membranes　in　this　process。　　Those　eggs　which　form　membranes　begin　to　develop；　but　perish　if　they　are　not　treated　with　hypertonic　sea…　water。　　Some　of　the　other　eggs；　however；　which　do　not　form　membranes；　develop　directly　into　normal　larvae　without　any　treatment　with　hypertonic　sea…water；　provided　they　are　exposed　to　the　blood　for　only　a　few　minutes。　　Presumably　some　blood　enters　the　eggs　and　causes　the　cytolytic　effects　in　a　less　degree　than　is　necessary　for　membrane…formation；　but　in　a　sufficient　degree　to　cause　their　development。　　The　slightness　of　the　cytolytic　effect　allows　the　egg　to　develop　without　treatment　with　hypertonic　sea…water。

Since　the　entrance　of　the　spermatozoon　causes　that　degree　of　cytolysis　which　leads　to　membrane…formation；　it　is　probable　that；　in　addition　to　the　cytolytic　or　membrane…forming　substance　（presumably　a　higher　fatty　acid）；　it　carries　another　substance　into　the　egg　which　counteracts　the　deleterious　cytolytic　effects　underlying　membrane…formation。

The　question　may　be　raised　whether　the　larvae　produced　by　artificial　parthenogenesis　can　reach　the　mature　stage。　　This　question　may　be　answered　in　the　affirmative；　since　Delage　has　succeeded　in　raising　several　parthenogenetic　sea…urchin　larvae　beyond　the　metamorphosis　into　the　adult　stage　and　since　in　all　the　experiments　made　by　the　writer　the　parthenogenetic　plutei　lived　as　long　as　the　plutei　produced　from　fertilised　eggs。

（c）。　　ON　THE　PRODUCTION　OF　TWINS　FROM　ONE　EGG　THROUGH　A　CHANGE　IN　THE　CHEMICAL　CONSTITUTION　OF　THE　SEA…WATER。

The　reader　is　probably　familiar　with　the　fact　that　there　exist　two　different　types　of　human　twins。　　In　the　one　type　the　twins　differ　as　much　as　two　children　of　the　same　parents　born　at　different　periods；　they　may　or　may　not　have　the　same　sex。　　In　the　second　type　the　twins　have　invariably　the　same　sex　and　resemble　each　other　most　closely。　　Twins　of　the　latter　type　are　produced　from　the　same　egg；　while　twins　of　the　former　type　are　produced　from　two　different　eggs。

The　experiments　of　Driesch　and　others　have　taught　us　that　twins　originate　from　one　egg　in　this　manner；　namely；　that　the　first　two　cells　into　which　the　egg　divides　after　fertilisation　become　separated　from　each　other。　　This　separation　can　be　brought　about　by　a　change　in　the　chemical　constitution　of　the　sea…water。　　Herbst　observed　that　if　the　fertilised　eggs　of　the　sea…　urchin　are　put　into　sea…water　which　is　freed　from　calcium；　the　cells　into　which　the　egg　divides　have　a　tendency　to　fall　apart。　　Driesch　afterwards　noticed　that　eggs　of　the　sea…urchin　treated　with　sea…water　which　is　free　from　lime　have　a　tendency　to　give　rise　to　twins。　　The　writer　has　recently　found　that　twins　can　be　produced　not　only　by　the　absence　of　lime；　but　also　through　the　absence　of　sodium　or　of　potassium；　in　other　words；　through　the　absence　of　one　or　two　of　the　three　important　metals　in　the　sea…water。　　There　is；　however；　a　second　condition；　namely；　that　the　solution　used　for　the　production　of　twins　must　have　a　neutral　or　at　least　not　an　alkaline　reaction。

The　procedure　for　the　production　of　twins　in　the　sea…urchin　egg　consists　simply　in　this：the　eggs　are　fertilised　as　usual　in　normal　sea…water　and　then；　after　repeated　washing　in　a　neutral　solution　of　sodium　chloride　（of　the　concentration　of　the　sea…water）；　are　placed　in　a　neutral　mixture　of　potassium　chloride　and　calcium　chloride；　or　of　sodium　chloride　and　potassium　chloride；　or　of　sodium　chloride　and　calcium　chloride；　or　of　sodium　chloride　and　magnesium　chloride。　　The　eggs　must　remain　in　this　solution　until　half　an　hour　or　an　hour　after　they　have　reached　the　two…cell　stage。　　They　are　then　transferred　into　normal　sea…water　and　allowed　to　develop。　　From　50　to　90　per　cent　of　the　eggs　of　Strongylocentrotus　purpuratus　treated　in　this　manner　may　develop　into　twins。　　These　twins　may　remain　separate　or　grow　partially　together　and　form　double　monsters；　or　heal　together　so　completely　that　only　slight　or　even　no　imperfections　indicate　that　the　individual　started　its　career　as　a　pair　of　twins。　　It　is　also　possible　to　control　the　tendency　of　such　twins　to　grow　together　by　a　change　in　the　constitution　of　the　sea…water。　　If　we　use　as　a　twin…producing　solution　a　mixture　of　sodium；　magnesium　and　potassium　chlorides　（in　the　proportion　in　which　these　salts　exist　in　the　sea…water）　the　tendency　of　the　twins　to　grow　together　is　much　more　pronounced　than　if　we　use　simply　a　mixture　of　sodium　chloride　and　magnesium　chloride。

The　mechanism　of　the　origin　of　twins；　as　the　result　of　altering　the　composition　of　the　sea…water；　is　revealed　by　observation　of　the　first　segmentation　of　the　egg　in　these　solutions。　　This　cell…division　is　modified　in　a　way　which　leads　to　a　separation　of　the　first　two　cells。　　If　the　egg　is　afterwards　transferred　back　into　normal　sea…water；　each　of　these　two　cells　develops　into　an　independent　embryo。　　Since　normal　sea…water　contains　all　three　metals；　sodium；　calcium；　and　potassium；　and　since　it　has　besides　an　alkaline　reaction；　we　perceive　the　reason　why　twins　are　not　normally　produced　from　one　egg。　　These　experiments　suggest　the　possibility　of　a　chemical　cause　for　the　origin　of　twins　from　one　egg　or　of　double　monstrosities　in　mammals。　　If；　for　some　reason；　the　liquids　which　surround　the　human　egg　a　short　time　before　and　after　the　first　cell…division　are　slightly　acid；　and　at　the　same　time　lacking　in　one　of　the　three　important　metals；　the　conditions　for　the　separation　of　the　first　two　cells　and　the　formation　of　identical　twins　are　provided。

In　conclusion　it　may　be　pointed　out　that　the　reverse　result；　namely；　the　fusion　of　normally　double　organs；　can　also　be　brought　about　experimentally　through　a　change　in　the　chemical　constitution　of　the　sea…water。　　Stockard　succeeded　in　causing　the　eyes　of　fish　embryos　（Fundulus　heteroclitus）　to　fuse　into　a　single　cyclopean　eye　through　the　addition　of　magnesium　chloride　to　the　sea…water。　　When　he　added　about　6　grams　of　magnesium　chloride　to　100　cubic　centimetres　of　sea…water　and　placed　the　fertilised　eggs　in　the　mixture；　about　50　per　cent　of　the　eggs　gave　rise　to　one…eyed　embryos。　　〃When　the　embryos　were　studied　the　one…eyed　condition　was　found　to　result　from　the　union　or　fusion　of　the