The distribution of special cytoplasmic differentiations of the egg during early cleavage in Limnaea stagnalis

[摘要] UncleavedeggsofLimnaeastagnalisshowapatternofcytoplasmicdifferentiationsinwhichthereare6“subcorticalaccumulations”(SCA)intheequatorialregion.SCAconsistofadensecytoplasmicmatrix,containingaspecialkindofsmallgranules.Atcertainstagesalsolens-shapedbodies,consistingofcoiledthreads,arefoundinthem.AttheuncleavedstagetheSCAformaregularpattern,whichisdorsoventralandnearlysymmetrical.Itsplaneofsymmetrycoincideswiththemedianplaneofthefutureembryo.Justbeforefirstcleavage,theSCAexhibitalatitudinalextensionandfusion.Theyaredistributedinaregularwayamongthe2,then4,blastomeres.Theirpositionsinthe4quadrantsoftheeggexhibitcharacteristicdifferences.Ineachquadranttheyshowagradualshifttowardthevegetativepole.Incentrifugedeggs,theSCAareindistinguishableimmediatelyaftercentrifuging.Inafewhours,however,theyarere-formedintheirtypicallocalization,evenincleavingeggs.Afterfirstandsecondcleavage,thespindleremnantsaredisplacedinavegetaldirectionintheinterblastomericwalluntiltheylieimmediatelybeneaththesurfaceinthemostvegetativepartofthiswall.TheSCAprobablyreflectparticularsofthecorticalmorphogeneticfield.Astheirpatternintheuncleavedeggisidenticalwiththepatternoffolliclecellssurroundingthegrowingoocyteinthegonad,theseobservationslendsupporttothehypothesisthatthecorticalfieldis“imprinted”upontheeggduringoogenesisbythesurroundingstructures.Itspatternshowsalterationsofadynamicnatureduringearlycleavage,probablyconnectedwithreconstructionprocessesinthecellmembraneatcelldivisions.

[学科分类] 生物科学（综合）