darwin and modern science-第79节

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　the　eggs　of　the　overwhelming　majority　of　animals　cannot　develop　unless　a　spermatozoon　enters　them。　　In　this　case　a　living　agency　is　the　cause　of　development　and　the　problem　arises　whether　it　is　possible　to　accomplish　the　same　result　through　the　application　of　well…known　physico…chemical　agencies。　　This　is；　indeed；　true；　and　during　the　last　ten　years　living　larvae　have　been　produced　by　chemical　agencies　from　the　unfertilised　eggs　of　sea…urchins；　starfish；　holothurians　and　a　number　of　annelids　and　molluscs；　in　fact　this　holds　true　in　regard　to　the　eggs　of　practically　all　forms　of　animals　with　which　such　experiments　have　been　tried　long　enough。　　In　each　form　the　method　of　procedure　is　somewhat　different　and　a　long　series　of　experiments　is　often　required　before　the　successful　method　is　found。

The　facts　of　Artificial　Parthenogenesis；　as　the　chemical　fertilisation　of　the　egg　is　called；　have；　perhaps；　some　bearing　on　the　problem　of　evolution。　If　we　wish　to　form　a　mental　image　of　the　process　of　evolution　we　have　to　reckon　with　the　possibility　that　parthenogenetic　propagation　may　have　preceded　sexual　reproduction。　　This　suggests　also　the　possibility　that　at　that　period　outside　forces　may　have　supplied　the　conditions　for　the　development　of　the　egg　which　at　present　the　spermatozoon　has　to　supply。　　For　this；　if　for　no　other　reason；　a　brief　consideration　of　the　means　of　artificial　parthenogenesis　may　be　of　interest　to　the　student　of　evolution。

It　seemed　necessary　in　these　experiments　to　imitate　as　completely　as　possible　by　chemical　agencies　the　effects　of　the　spermatozoon　upon　the　egg。　When　a　spermatozoon　enters　the　egg　of　a　sea…urchin　or　certain　starfish　or　annelids；　the　immediate　effect　is　a　characteristic　change　of　the　surface　of　the　egg；　namely　the　formation　of　the　so…called　membrane　of　fertilisation。　　The　writer　found　that　we　can　produce　this　membrane　in　the　unfertilised　egg　by　certain　acids；　especially　the　monobasic　acids　of　the　fatty　series；　e。g。　formic；　acetic；　propionic；　butyric；　etc。　　Carbon…dioxide　is　also　very　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