イトミミズ卵の受精
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概要
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Morphological changes of fertilization were investigated in the eggs of a freshwater oligochaete, Tubifex hattai. The fertilizing spermatozoa are discharged from the spermathecae into the cocoon tube when the tube passes on the opening of spermathecae at the time of cocoon deposition. At 30 minutes after oviposition, the first morphological change of fertilization is detected on the egg surface near the vegetal pole where a bulge occurs. The bulge becomes remarkable and is soon transformed into a small cylindrical process of the egg. This is fertilization cone and contains the penetrated spermatozoon. The cone regresses after about 30 minutes. At the time of fertilization, the egg possesses the first maturation metaphase spindle at the animal pole. The first and second polar bodies are extruded from the egg at about 3 and 5 hours after oviposition, respectively. The maternal chromosomes remained in the egg after meiosis begin to convert into vesicular pronucleus. Together with this, the sperm head of the compact condition becomes vesicular and advances toward the deeper part of the egg accompanying sperm aster. The meeting of the egg and sperm pronuclei takes place within 8.5 to 9 hours after oviposition. When the first cleavage spindle is formed, asters existed on both ends of the spindle are unequal in size, one being larger than the other. Furthermore, the egg lacks yolk granules in the restricted areas of the animal and vegetal poles where are known as "pole plasms". The vertical first cleavage divides the egg into unequal two blastomeres. The larger blastomere arises from the side of the spindle accompanying the larger aster and contains nearly all of "pole plasms".
- 1968-11-15